GB2166660A - Filtering and washing cellulosic pulp - Google Patents

Abstract

A slurry of cellulosic pulp stock and pulping liquor is distributed from headbox 6 to form a mat on the upper surface of an endless porous belt 2 and after initial drainage of liquid a second moving endless porous belt 3 is brought into contact with the upper surface of the mat. The two belts move in a converging path over individual groups of rollers to extract additional liquid from the mat. The mat contained between the belts is then passed through a series of washing stations where water is applied to the mat by distributors 60 and sucked off as the belts pass round suction rolls 47. Fresh water is delivered to the mat at the downstream station and the water is cascaded by gravity in countercurrent flow through the other washing stations. Following the final washing station, the belts are separated and the mat is removed from between the belts. <IMAGE>

Description

SPECIFICATION Method and Apparatus for Treating a Slurry Background of the Invention In the papermaking industry wood chips are digested at elevated temperatures with a pulping liquor. In the pulping process, cellulosic bonding ingredients are dissolved in the pulping liquor, and after pulping, it is desirable to recover the spent liquor from the pulp and process the liquor to recover its heat values, as well as the digesting chemicals. It is desired that the spent liquor be recovered with as high a solids content as possible, since the initial stage of the recovery procedure is the concentration of the liquor by evaporation.

Thus, to provide the most effective recovery process, a minimum quantity of wash water should be used to wash the pulp.

Traditionally the most common procedure for recovering the pulping liquor has involved the use of rotary drum vacuum filters. As one filtration does not ordinarily sufficiently wash the pulp, the pulp is typically reslurried with wash water and refiltered.

This sequence is repeated, usually in countercurrent fashion, to obtain the washing of the pulp and the desired recovery of the pulping liquor.

Rotary filter drums require substantial amounts of water to wash the pulp. This is due to the necessary reslurrying operations and due to the fact that drum filters require very low slurry consistencies for efficient operation. Thus, recovery of the pulping liquor from the washing process utilizing rotary vacuum drum filters requires substantial energy to concentrate the diluted pulping liquor in the recovery operation.

To eliminate the difficulties encountered with rotary drum filters, attempts have been made to filter the slurry on a moving horizontal belt-type filter. For example, United States Patent No.

4,046,521 shows a system in which the diluted slurry is deposited on a moving horizontal filter belt which passes over a series of suction boxes. Wash water is applied to the downstream suction box and the water passing through the mat supported on the horizontal belt is fed countercurrently through the various suction boxes to the upstream end of the system.

U.S. Patent 4,154,644 also discloses a pulp washing system in which the diluted pulp slurry is applied to the upper surface of a moving horizontal filter belt. In patent 4,154,644, a hood is sealed over the porous endless belt and the belt passes over a series of suction boxes and the liquid withdrawn from the slurry is recycled countercurrentlyto preceding sections of the washer. Gases and vapors drawn into the suction boxes with the wash liquid are separated from the liquid and recycled to the hood to control the atmosphere in the hood and maintain the desired pressure differential for operation of the washer.

As the moving horizontal belt that supports the pulp mat travels over a series of vacuum or suction boxes in the systems shown in Patents 4,046,621 and 4,154,644, there is a substantial frictional resistance to movement of the belt. The increased frictional resistance requires the drive system to have a substantially greater power requirement and also causes wear of the filter belt, reducing the service life of the belt.

U.S. Patent 4,160,297 and 4,246,669 show pulp washing systems in which a layer or mat of pulp is contained between two endless filter belts. In these Patents, horizontal drums are each mounted in a wash tank containing the wash liquid and the pulp mat contained between the filter belts passes under each of the wash drums and through the liquid contained in the tank. The wash liquid from the tank can pass inwardly through the pulp mat contained between the belts, or alternately, can flow outwardly from the drum through the mats into the tank. A countercurrent wash liquid system is employed in which the wash liquid passes between the wash tanks in a countercurrent manner by gravity flow.

Summary of the Invention The invention is directed to an improved method and apparatus for treating a liquid slurry containing solid materials, such as a slurry composed of cellulosic pulp stock and digestor or pulping liquor.

The flowable cellulosic slurry is distributed as a layer or mat onto the upper surface of a moving endless porous belt and after initial drainage of liquor from the mat, a second moving endless porous belt is brought into contact with the upper surface of the mat. The two belts move in a converging path over individual groups of rollers to extract liquor from the mat.

The mat contained between the belts is then passed through a series of conditioning stations where the pulp mat is subjected to dilution by washing liquid and drainage cycles for washing.

Fresh water is delivered to the the mat at the final downstream station and the washing liquid after passing through the mat is cascaded by gravity in countercurrent flow through the various conditioning stations.

Following the final conditioning station, the belts are moved in diverging paths and the mat is separated from the belts.

As the pulp mat is contained between the two moving belts, the position of the mat is fully controlled during the entire washing operation.

As the system utilizes no stationary vacuum boxes there is no vacuum frictional resistance to belt travel. This not only reduces the power requirements for operating the belts, but also provides longer belt life.

As a further advantage, the reduced frictional resistance and corresponding decreased belt wear enables a spliced belt to be utilized rather than a woven endless belt. With the use of a spliced belt, the belt can be installed on the machine and the free ends of the belt spliced together. With the use of a prefabricated endless belt, it is necessary to cantilever the rolls in orderto install the belt, and the cantilevered construction greatly increases the overall cost of construction of the washer.

As the pulp web is fully contained between the moving belts, the conditioning stages can be in a cascade configuration so that the wash water can flow countercurrently from station-to-station by gravity. This not only eliminates the need for pumping the wash liquid, but also reduces foaming of the liquid.

The washing system of the invention uses significantly less wash water than the rotary drum systems, so the digesting liquor can be recovered at a substantially lower cost. Alternately, utilizing the same power source, a greater number of conditioning stations can be utilized in the washer.

Other objects and advantages will appear in the course of the following description.

Description of the Drawings The drawings illustrate the best mode presently contemplated of carrying out the invention.

In the drawings: Figs. and 1B are a schematic representation of the pulp washing system of the invention; Fig. 2 is an enlargement of the initial liquor extraction section of the washing system shown in Fig. 1; Fig. 3 is an enlarged side elevation showing a pair of the conditioning stations; Fig. 4 is a side elevation of a pressure plate; Fig. 5 is a perspective view of the finned roller; Fig. 6 is a fragmentary enlarged transverse section of the finned roller and wash liquid distributing unit; Fig. 7 is an enlarged fragmentary longitudinal section of the finned roller; Fig. 8 is a side elevation of a modified form of the invention in which the upper belt is separated from the mat prior to approaching the conditioning station;; Fig. 9 is a side elevation of a modified form of the initial liquor extraction section; Fig. 10 is a side elevation of a modified form of the pressure plate utilizing a travelling belt; Fig. 11 is a modified form of the conditioning station utilizing fluid pressure to extract liquid from the web; Figs. 12A, 12B and 12C are a schematic representation of a modified and preferred form of the invention; and Fig. 13 is a section taken alone line 13~13 of Fig.

12.

Description of the Illustated Embodiment Fig. is a schematic representation of an apparatus for treating a liquid slurry containing solid material and more particularlyforwashing paper pulp stock. The apparatus includes a frame or supporting structure 1 and a pair of porous, filtering belts 2 and 3 are each mounted to travel in separate endless paths on the supporting structure 1.

In accordance with the invention, a layer or mat of pulp 4 is contained between the two filter belts 2 and 3 and the mat is conditioned or washed as it passes through the system to recover the pulping liquor.

The lower filtering belt 2 is mounted on a tensioning roll 5 and then passes beneath a headbox 6 where a layer of the pulp in the form of a mat 4 is applied to the upper surface of belt 2. Belt 2 supporting mat 4, then passes through an initial extraction section 7 where a portion of the pulping liquor is extracted from mat 4 and then passes through a treating or conditioning section 8 where the mat is washed with washing liquid to remove the pulping liquor. After passing through the conditioning section 8, the lower filter belt 2 passes around a driven couch roll 9 and then in the return run travels over a series of supporting rolls 10, 11, 12 and 13.Rolls 1S13 are suitably journalled for rotation in supporting structure 1 and roll 12 is mounted to the supporting structure in a manner to permit it to tilt in a horizontal plane. Adjustment of roll 12 operates in a known manner to maintain the alignment of the belt on the various rolls and prevent the belt from running off to one side or the other.

Tensioning roll 5 can be moved horizontally to provide the proper tension on belt 2. In this regard, the tensioning roll 5 is journalled in a carriage 14 which is movable in tracks on the supporting structure 1. A series of jacks, not shown, can be used to adjust the position of the carriage 14 relative to the frame to maintain the desired tension on belt 2.

The upper filter beit 3 is mounted over a tensioning roll 15, similarto tensioning roll 5, and then travels over a pair of guide rolls 16 and 17 which are journalled on supporting structure 1. As shown in Fig. the upper belt 3 moves in a converging path in the section 7 with respect to belt 2 to thereby squeeze the mat 4 contained therebetween to extract liquor from the mat.

After passing through the conditioning section 8, upper belt 3 is trained over a driven couch roll 18 and then passes in a return run over rolls 19,20,21 and 22 in returning to the tensioning roll 15. As in the case of roll 12, roll 22 can be mounted to tilt in a horizintal plane to thereby maintain alignment of belt 3 on the various rolls.

Tensioning roll 15 is mounted for rotation on a carriage 23, similar to carriage 14,which is movable on tracks carried by supporting structure 1. Jacks, not shown, interconnect carriage 23 with structure 1 and enable the carriage to be moved relative to supporting structure 1 to thereby provide the desired tension on the upper belt 3.

The belts 2 and 3 are porous and are preferably formed of plastic or metal screening to enable the washing liquid to readily penetrate through the belts.

As best shown in Fig. 2, the pulp slurry, which can be diluted with water to a flowable consistency, is discharged by gravity through a transverse slot 24 in the lower surface of head box 6 onto the upper surface of belt 2. Located beneath the head box 6 is a forming board 25 having a series of slots 26 and a portion of the liquid contained in the slurry deposited on belt 2 can drain through slots 26 into trough 27 which is mounted on the lower portion of supporting structure 1. The liquid being discharged into trough 27 can be pumped from a drain outlet 28 in the trough to a recovery operation.

The initial extraction section 7 includes a series of parallel spaced rolls 29, each of which is journalled for rotation on the supporting structure 1. The belt 2 is supported on the rolls 29 as the belt moves through the section 7. Liquid deflecting plates 30 are mounted between adjacent rolls 29 and the upper ends of plates 30 are provided with bent ends 31 which extend in an upstream direction. Deflector plates 30 prevent the liquid from being thrown from one roll to another as the liquid drains from mat 4 as the mat passes over the rolls 29.

The initial extraction section 7 also includes an upper roll assembly 32 composed of a frame 33, one end of which is pivoted to a cross beam 34 at pivot 35, so that the frame can be pivoted in a vertical direction with respect to the supporting structure 1.

To pivot the frame, a pair of jacks 36 are rotatably connected to the opposite end of frame 33 and each jack is threaded within a nut 37 mounted on cross support 38 that is secured to supporting structure 1.

By threading jacks 36, frame 33 can be moved in a vertical path around the pivot 35.

As shown in Fig. a series of rolls 39 are journalled in brackets 40 that are attached to frame 33. Each roll is mounted to the respective brackets 40 through an adjusting mechanism, which permits the roll to be adjusted vertically with respect to the brackets 40. The adjusting mechanism permits the rolls 39 to be individually adjusted with respect to the supporting frame 33.

Frame 33 is adjusted through operation of jacks 36 so that the upper filter belt 3, which rides against the rolls 39, will move in a converging path toward the lower belt 2, thus squeezing the mat 4 between the two belts and extracting liquid from the pulp mat. As shown in Fig. 1 the upper rolls 39 are located in alignment with the spaces between the lower rolls 29 and the downstream rolls 39 are partially disposed within-the spaces between lower rolls 39, so that the belts 2 and 3, containing mat 4, move in an indulating path through the downstream portion of section 7. The undulating path serves to alternately compress and release the mat, thereby aiding in extraction of the liquid from the mat.

The treating section 8 is composed 6#a a series of conditioning stations 42,43,44, and 45 and the pulp mat is subjected to water dilution and drainage in each of the stations. While Fig. 1 shows four stations being utilized, it is contemplated that any number of stations can be employed depending upon the particular operation involved.

Each of the stations 42~45 is similar in construction and therefore the description will be directed to station 45, it being understood that the construction of stations 42~44 would be the same.

As best illustrated in Fig. 3, station 45 includes a roll 46 that is journalled in supporting structure 1 and the belts 2 and 3 containing mat 4 pass beneath roll 46 and then over a finned roll 47. As best shown in Figs.4~6, roll 47 is composed of a cylindrical drum 48 which is mounted on shaft 49. A plurality of radially extending fins 50 extend outwardly from drum 48 and define a series of compartments 51.

End rings 52 are secured to the respective ends of the fins 50 and are spaced radially outward of drum 48.

The roll 47 also includes a pair of end headers 53 and shaft 49 extends outwardly through the respective headers 53. One end of shaft 49 is connected to a suitable drive mechanism, not shown, so that the shaft 49 and drum 48 will rotate relative to the fixed headers 53. Suitable seals are positioned between the mating surfaces of the drum 48 and rings 52 and the header 53 to maintain the seal between the members as the drum is rotated.

The interior of each header 53 is connected through a conduit 54 to a suitable source of vacuum, such as a vacuum pump 55, and the inner surface of each header is provided with a slot 56 that extends through an arc of about 90 and communicates with the space between the end ring 52 and the drum 48.

Conduit 54 is connected to the suction side of a vacuum pump 55 while the discharge side of pump 55 is connected through conduit 57 to a collection trough 58. With this construction, vacuum or reduced pressure is applied to each header 53, and the vacuum acts through each slot 56 to the compartments 51 as the roll 47 rotates to thereby draw wash liquid from the portion of the mat 4 passing over roll 47.

A liquid distributing unit 60 is mounted above the finned roll 47 and acts to feed fresh wash water in the form of a curtain onto the mat 4, contained between belts 2 and 3, as it passes over the roll 47.

As shown in Fig. 6, the liquid distributing unit 60 inlcudes a housing 61 and an inlet pipe 62 is connected to the housing to supply fresh water to the housing. An upstanding weir 63 is mounted centrally within housing 61 and the water will flow over the weir 63 and down an inclined plate 64 onto the mat 4 as it moved away from the finned roll 47.

The wash liquid flowing through mat 4 from distributing unit 60, as well as the liquid drawn from the mat by vacuum pump 55, is delivered to trough 58. From the trough 58 the liquid will be transferred by gravity through a drain line 66 to the next upstream liquid distributing unit 60, as best illustrated in Figs. 1 and 3, which is located at a lower level than the bottom of the trough 58, as shown in Fig. 3. As the central portion the drain line 66 is below the level of the next succeeding distributing unit 60, as shown in Fig. 3, no air will be drawn from the trough 58 to the preceding unit 60.

In a similar fashion, the wash liquid will be transferred by gravity flow from each conditioning station to the next preceding station, and from upstream station 42, the liquid will flow to trough 27 for final discharge through drain line 28. This cascade distribution system eliminates the need for pumps to pump the liquid in a counterflow manner through the washer.

In addition, to the pressure differential associated with finned roll 47 to extract liquid from the mat 4, a pressure plate assembly 67 is located immediately downstream of the roll 47 and acts to further extract liquid from the pulp mat 4. As shown in Fig. 4, the pressure plate assembly 67 includes a flexible metal plate 68 having one end secured to a leg 69 that extends downwardly from generally horizontal beam 70. A series of adjusting rods 71 have their lower ends connected to plate 68 and rods 71 are threaded within nuts 72 carried by beam 70. By threading rods rolative to nuts 72, plate 68 can be flexed to provide the desired configuration or contourforthe plate 68.

An additional adjustment is provided through jacks 73 which are connected to the beam 70 and are threaded within turnbuckles 74. Through operation of jacks 73, the angularity of beam 70 and plate 68 can be adjusted.

As shown in Fig. 4, the plate 68 is normally disposed at a shallow acute angle with respect to the path of travel of the belts 2 and 3 that contain the mat 4. This converging angle acts to squeeze the mat 4 and further extract liquid from the mat.

After passing through the final conditioning station 45, the composite belt structure containing mat 4 passes over a pair of rolls 75 and 76 which are suitably journalled with respect to supporting structure 1 and then passes over a vacuum box 77 which is connected through line 78 to a suitable source of sub atmospheric pressure orvacuum. The upper surface of vacuum box 78 contains a series of perforations or slits and the vacuum serves to draw the mat 4 against the lower belt 2 just prior to the separation of the belts 2 and 3 at the end of their operating runs.

An air distributing conduit 79 which is connected by line 80 to a suitable source of air under pressure, ejects a curtain of high pressure air or other gas against the upper surface of belt 3 to further aid in removing the mat 4from the upper belt.

With the mat 4 adhered to the lower belt, the lower belt passes around the couch roll 9 where the mat 4 will normally be separated by gravity from the lower belt. A second air discharge conduit 81 can be mounted adjacent roll 9 and conduit 81 is connected through line 82 to a suitable source of air under pressure. Air is discharged from conduit 81 against the belt 2 will act to remove the web 4 from the belt in the event the web had not separated from the belt by gravity as the belt 3 passed around roll 9.

A suitable doctor blade 83 can be mounted to ride against the surface of roll 10 to remove any cellulosic material that may adhere to the roll 10. In addition, a cleaning water shower 84 can be mounted adjacent roll 11 to spray a curtain or film of water on the roll 11 and belt 3 to aid in cleaning the roll and belt.

Similarly, a water shower 85 and doctor blade 86 can be associated with roll 21 to clean the upper belt 3 and roll 21 of any cellulosic material.

Fig. 11 illustrates a modified form of a conditioning station 42-45 in which pressure is utilized to remove the liquid from the mat 4, as opposed to vacuum. The modified station as illustrated in Fig. 11, includes a roll 87, similar in function to roll 46 and the composite belt structure, composed of belts 2and 3 containing the mat 4, passes upwardly over roll 87, over roll 88 and then downwardly around finned roll 89, which is similar in construction to roll 47. Roll 89, as previously described, includes a pair of headers 90 and the lower portion of each header is provided with a curved slot 91 that extends through approximately SOC and communicates with the compartments 92 located between fins 93 of roll 89.Each header 90 is connected via conduit to a suitable source of air under pressure, so that the pressurized air entering headers 90 will pass through slots 91 into the compartments 92 between fins 93 and then outwardly through the mat 4.

Washing liquid is supplied to the roll 89 by a liquid distributing unit 95, similar in construction to unit 60. In this form of the invention, the liquid distributing unit 95 supplies a stream of washing liquid into the compartments in the upper portion of roll 89. As the roll 89 rotates, the water contained within the compartments flows outwardly into the mat 4 and as the mat passes in registry with the slot 91, the air pressure will drive the water outwardly through the mat, where it will be collected in the trough 58.

Fig. 8 illustrates a modified form of a conditioning station 42-45 in which the upper filter belt 3 is separated from the mat 4 when the washing liquid is applied to the mat. In this embodiment, the composite structure, i.e. mat 4 contained between belts 2 and 3, passes around roll 96, similar in function to roll 46, and the lower belt 2, along with mat 4, then passes over the finned roll 97, similarto roll 47, previously described. The separated upper belt 3 travels around rolls 98 and 99, which are suitably journalled in supporting structure 1 and then returns to engagement with the mat 4 at a location immediately downstream of roll 97.

Washing liquid is supplied to the mat 4 through a water distributing unit 100, similar to water distributing units 60, previously described, and the water will flow downwardly through the upper belt 3, which at this time is not in contact with mat 4, into the mat 4 and the water will then be drawn through the mat and the lower belt 2 by the vacuum applied to the finned roll 97.

A pressure plate assembly 101, similar to pressure plate assembly 67, can be mounted immediately downstream of roll 97 to further air in extracting liquid from the mat 4.

Fig. 9 illustrates a modified form of the initial extracting section 7 in which plates are employed in place of the roller banks 29 and 39. In this embodiment, the four downstream rolls 29 are replaced by a perforated plate 102 having a series of perforations or holes 103 through which the liquid extracted from mat 4 can drain. Located above the plate 102 is a second solid plate 104 which is mounted at an angle with respect to plate 102 to provide a converging path for the composite belt structure. With the use of the spaced rolls 39, as shown in Fig. 1, there may be a tendency for the liquid to bubble up between the rolls 39 and pass transversely to the side edge of the belt 3. The embodiment shown in Fig. 9 prevents this action due to the use of the solid, non-perforated upper pressure plate 104.

Fig. 10. illustrates a modified form of the pressure plate assembly 67. In the construction shown in Fig.

10, the pressure plate assembly 67 is incorporated with a movable endless belt 105 which is mounted on a pair of rolls 106 and 107 that are journalled in the supporting structure 1. As the belt 105 can move in an endless path around rolls 106 and 107, the presure plate assembly of Fig. 10 provides less frictional resistance to the movement of the belts 2 and 3 and thereby aids in reducing the power requirement for the system. The flexible plate 68 can be adjusted in contour, as previously described, and the belt 105 will conform to the contour of plate 68 to provide the desired path of travel for the belts 2 and 3.

Figs. 12 and 13 illustrate a modified and preferred form of the pulp washer of the invention. The washer includes a frame or supporting structure 108, and a head box 109, containing a cellulosic slurry, is mounted on frame 108 and the vertical position of the head box can be adjusted through adjusting rods 110 which connect the forward end of the head box to frame 108. See Fig. 12B.

A weir 111 is located in headbox 109 and the vertical position of the weir can be adjusted through a pair of adjusting rods 112.

The paper stock contained in the head box 109 has a consistency of about 2% solids and is discharged through an outlet 113 onto a porous endless belt 114, similar in construction to belts 2 and 3 of the first embodiment, in the form of a layer or mat 115.

Belt 114 carrying mat 115 passes over a forming board 116 having a plurality of slits or perforations 117 and a portion of the liquid is drained from the mat 115 through the slits 117.

After leaving the forming board 116, the filter belt 114 carrying the mat 115 passes over a bank of rolls 118, which are disposed in a generally curved or convex configuration, as illustrated in Fig. 1 2B. The shafts of rolls 118 are journalled within bearing blocks 119 that are supported on curved beam 120 that extends between vertical supports of frame 108.

To confine the side edges of mat 115 as the belt 114 travels overtheforming box 116 and the first group of rollers 118, side plates 120e are secured to frame 108 and are located along the sides of the path of travel of belt 114.

Due to the curved configuration of the bank of rolls 118, the belt 114 carrying mat 115 will be partially wrapped around the periphery of each roll 118 and this will aid in the extraction of liquid from the mat.

To further aid in the extraction of liquid from mat 115, it is contemplated that one or more of the rolls 118, such as the second and third rolls from the upstream end, can be power driven at a higher rate of speed than the travel of belt 114. This variation in speed causes a vacuum pulse at the downstream point where the belt leaves the roll to aid in dewartering of the mat.

As in the case of the first embodiment, liquid deflecting plates 121 are mounted between adjacent rolls 118 and the upper ends of plates 121 are provided with bent ends which extend in an upstream direction. Plates 121 prevent the liquid from being thrown from one roll to another as the liquid drains from mat 115.

As in the case of the first embodiment, an upper porous filter belt 122 is moved in a converging path with respect to belt 114 to thereby squeeze the mat 115 contained therebetween to extract liquid from the mat. As shown in Fig. 12B, upper belt 122 passes around an adjustable roll 123 which is journalled on the ends of a pair of arms pivotable 124. Arms 124, in turn, are pivotally connected to frame 108 by pivot 125. Arms 124 can be pivoted relative to the frame by a pair of jacks 126 which interconnect the outer portions of arms 124 and frame 108. By proper adjustment of jacks 126, the angle of convergence between the belts 122 and 114 can be varied, as desired.

Belt 122 is supported upstream from adjusting roll 123 by roll 127 which is journalled on frame 108.

Due to the position of roll 127, uniform tension can be maintained on belt 122 during adjustment of roll 123.

Liquid extracted from the mat through the forming board 116 and through the upstream section of the bank of rollers 118 is drained into a tank 128 mounted on frame 108, while the liquid extracted from the downstream portion of the bank of rolls 118 is collected in a second tank 129 which is located side-by-side with tank 128, as shown in Fig.

12B. Suitable baffles 130, 131 and 132 are positioned in the tanks 128 and 129 to gently drain the liquid to the bottom of the tank and minimize foaming.

Liquid in tank 128 can be discharged through an outlet 133, and a portion of the liquid discharged through outlet 133 can be used for stock dilution while a second portion can be treated to remove fibers and thereafter fed to the evaporators for concentration. Similarly, the liquid in tank 129 can be discharged through an outlet 134 to the evaporator and subsequently to chemical conversion.

Belt 114 supporting mat 115 initially travels in an uphill path over rolls 118 which acts to retard the velocity of the stock issuing from the headbox to aid in initial liquid extraction. Mat 115 is then squeezed between the converging belts 114 and 122 to further remove liquid from the mat, and the composite structure 135, consisting of belt 114, mat 115 and belt 122, then passes over the central portion of the bank or rolls 118 and then to the downstream portion which has a greater curvature, enabling the composite structure to wrap further around the rolls to assist in the removal of liquid from the mat 115.

After leaving the downstream roll 118, the composite structure 135 passes under a doctor blade 136, as shown in Fig. 12C, which serves to doctor liquid from the upper surface of belt 122. The liquid is collected in a pan 137 mounted on frame 108, and liquid collected in pan 137 is drained through a drain line 138 to tank 129.

The composite structure 135 then passes under a dewatering roll 140 journalled on frame 108 and having a rubber or resilient surface. The composite structure 135 is wrapped through an are of greater than 20 , and preferably about 50 , around roll 140 and this compressive action will furtherdewaterthe mat 115, so that the mat leaving the roll 140 will have a solids content generally in the range of 12% to 14%.

Water extracted from the mat 115 as the composite structure 135 passes around roll 140 is collected in a tank 141 and an inclined baffle plate 142 is mounted in tank 141 to minimize foaming of the liquid discharge into the tank. Liquid contained in tank 141 can overflow through outlet 143 into tank 129.

Mat 115 contained between belts 114 and 122 is then subjected to a series of washing steps by passing through a plurality of washing stations indicated by 144--149. Washing stations 144~149 are similar in construction so that only the washing station 144 will be described in detail. Washing station 144, as illustrated in Fig. 12C, includes a trough 150 having an inlet 151. A perforated plate 152 is mounted transversely across the bottom of trough 150 and an adjustable weir 153 is located adjacent the plate 152. Wash liquid entering trough 150 through inlet 151 will pass through the perforated plate and over the weir 153 and onto the composite structure 135.

Wash station 144 also includes a pair of side walls 154 which extend downstream from the weir 153 and act to retain the wash liquid on the surface of the composite structure as the composite structure moves in an upwardly inclined path. This is best illustrated in Fig. 13.

It is preferred that the composite structure travel in an inclined path to the horizontal, generally about 50 to 30 to the horizontal and preferably about 10 , as the wash liquid is applied to its upper surface.

The inclined path provides greater penetration of the liquid into the mat 115 for a given length of travel. If the incline is too steep, the liquid may flow rearwardly without properly penetrating the mat.

On the other hand, if the composite structure 135 is travelling in a horizontal path as the wash liquid is applied, the length of the wash station would have to be increased to obtain the desired liquid penetration into the mat 115.

After passing through the wash station 144, the composite structure 135 then passes around a dewatering compression roll 155 where liquid is extracted by compression from mat 115. The extracted liquid is drained into a pan 156 having an outlet 157 which is connected to drain line 158. The liquid being collected in pan 156 is the final wash liquor and is delivered through the line 158 to the evaporators and to further chemical conversion.

After passing overthe dewatering roll 155, the composite structure 135 travels over a turning roll 160 which is suitably journalled on frame 108 and then enters the second washing station 145 where wash liquid is applied to the upper surface of the composite structure 135 moving in an upwardly inclined path to thereby impregnate mat 115 with the wash liquid, as previously described.

A composite structure 135 then passes over a pair of dewatering rolls 162 and 163 which are suitably journalled on frame 108. Water extracted from the mat 115 is collected in trough 164 and the liquid is then conducted by gravity through an outlet 165 in trough 1 64 through line 166 to the inlet 151 of the wash station 144, as shown in Fig. 12C.

After passing over the dewatering rolls 162 and 163, wash liquid is again applied to the composite structure 135 in wash station 146, as the structure is moved in an upwardly inclined path. Composite structure 135 then passes over a pair of dewatering rolls 167 and 168 which are suitably journalled on frame 108 and liquid extracted from mat 115 is collected in trough 169. Trough 169 is provided with an outlet 170 and line 171 connects outlet 170 to the inlet of the trough 150 of wash station 145.

This process is repeated with the wash liquid being extracted in each wash station being delivered by gravity to the next upstream wash station. Fresh wash water is introduced to the inlet 151 of wash station 149 through line 171a.

After passing through wash station 149, the composite structure 135 passes over a compression dewatering roll 172 where water is extracted from the mat and is delivered, as previously described, to the next succeeding upstream wash station.

After travelling over roll 172, composite structure 135 is trained over a drive roll 173 which can be driven in any suitable manner. Drive roll 173 serves to drive composite structure 135 through the machine.

As shown in Fig. 1 2A, belt 122 carrying mat 115, is separated from belt 114 as the belts leave drive roll 173. Belt 122 and mat 115 pass downwardly across an air nozzle 174 which acts to separate mat 117 from belt 122. Belt 122 then passes under a roll 175 and the separated mat 115 falls by gravity into a bin 176 where it is broken up and discharged from the bin by an auger 177. Belt 122 then passes over a roll 178 and a doctor blade 179 is associated with roll 178 and serves to remove the mat 115, if for some reason it has not dropped into bin 176 and has adhered to roll 178.

Following this, the belt 122 passes under a fresh water cleaning shower which acts to clean the belt and the cleaning water is collected in pan 181 having an outlet 182. Following cleaning of belt 122, the belt passes around roll 183 and under a stretch roll 184 which is adjustable in a horizontal direction with respect to frame 108 to maintain proper tension on belt 122. Belt 122 then passes over guide roll 185 to roll 127 and then to the adjustable roll 123, as previously described.

After being separated from belt 122, belt 114 passes under an air nozzle 186, as shown in Fig. 12A, which will act to blow the mat 115 from the underside of belt 144, if it had adhered to that belt.

Belt 114 then passes over a second drive roll 187 having a doctor blade 188 which scrapes the mat from the drive roll 187, if the mat had accidentally been transferred to that roll.

Belt 114 then passes downwardly into contact with a lip 189 formed on the edge of bin 176. If, for some reason, the mat had adhered to the belt 114, the lip 189 will scrape the mat from the belt and deposit it in bin 176.

Following this, belt 114 passes adjacent a cleaning water spray 190 and then over a roll 191. Water from the cleaning shower is collected in a pan 192 having an outlet 193.

Belt 114 then travels around a pair of rolls 194 and 195 and then over a tensioning or stretch roll 196 which is mounted for horizontal movement with respect to frame 108. After passing over stretch roll 196, belt 114 travels over rolls 197, 198 and 199 and then to the forming box 116, as previously described.

With the washer as shown in Figs. 12 and 13, the mat 115, contained between the two filter belts 114 and 122, is passed in a curved path over the bank of rolls 118. The curved path enables the web to be partially wrapped around each roll and thus the web is subjected to slight compression as it moves in the path to increase the effectiveness of the dewatering.

The system also provide more effective washing by passing the composite structure 135 in an upwardly inclined path beneath a puddle of the wash liquid. This arrangement improves the liquid penetration into the mat for a given length of travel of the composite structure.

In the system shown in Fig. 12, the return runs of the endless belts 114 and 122 are utilized for the wash stations, thereby reducing the overall length of the system and minimizing tensioning and guide problems belts.

In addition, with the construction shown in Fig. 12 the drainage is simplified, in that there are no return runs of the belts that interfere with the drainage system.

With the pulp washing system of the invention, the pulp mat is fully contained between the porous filter belts throughout the entire cycle of operations, and the mat can move through any desired angular path of travel. This enables the wash or conditioning stations to be located at different elevations, thereby permitting the wash liquid to be transferred countercurrently by gravity from station to station.

By utilizing gravity flow, expensive pumping equipment is eliminated, as well as minimizing foaming which normally will accompany a pumping operation.

The wash liquid is extracted in the system of the invention by passing the composite filter belt structure over rotatable rolls. As the rolls are rotatable there is less frictional resistance to movement of the filter belts and this results in longer fabric life, as well as reduced power requirements for driving the belts. Reducing the power requirement means that a smaller capacity drive can be utilized for the same number of conditioning stations, or alternately, a greater number of conditioning stations can be utilized for the same capacity drive.

As there is reduced frictional resistance and tension on the fabric belts, seamed belts can be employed, as opposed to endless woven belts. With the use of seamed belts, the belt can be installed on the machine and the free ends then seamed together. With the use of a fabricated endless belt it is necessary to employ cantilevered rolls in order to install the belt on the machine and the cantilevered construction substantially increases the overall cost of the machine.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particulary pointing out and distinctly claiming the subject matter which is regarded as the invention.

Claims (32)

1. A method of treating a liquid slurry containing solid material, comprising the steps of applying a layer of the slurry to the upper surface of a moving first porous belt to form a mat, bringing a second moving porous belt into contact with the upper surface of the mat to sandwich the mat between said belt and provide a composite belt structure, moving the belts in a converging path to extract liquid from the mat, passing the compcsite structure in a curved path over at least one roll to further extract liquid from the mat, applying wash liquid to the upper surface of said composite structure with said liquid penetrating the mat contained between said belts, removing the wash liquid from the mat by passing the composite structure around a rotatable roll, and thereafter separating the mat from between said belts.

2. The method of claim 1, wherein said step of passing the composite structure in a curved path comprises passing the structure over the series of generally parallel rolls disposed in a curved configuration with said composite structure partially wrapping around each roll as it passes thereover.

3. The method of claim 1, and including the step of moving said composite structure in an upwardly inclined path of travel as the washing liquid is applied to the upper surface of said structure.

4. The method of claim 3, wherein said washing liquid is applied in a form of a liquid body to the upper surface of said structure.

5. The method of claim 3, wherein said composite structure is moved in an upwardly inclined path of 5 to 30 with respect to the horizontal.

6. The method of claim 1, wherein the step of removing the wash liquid comprises contacting the composite structure with the periphery of the roll through an arc of over 20 .

7. A method of treating a liquid slurry containing solid material, comprising the steps of applying a layer of a liquid slurry of containing solid material to the upper surface of a moving first porous belt to form a mat, bringing a second moving porous belt into contact with the upper surface of said mat to sandwich the mat between said belts and provide a composite belt structure, passing the structure over a series of parallel rolls disposed in a curved configuration and partially wrapping said structure around each roll as it passes thereoverto extract liquid from said mat, applying wash liquid to the upper surface of the composite structure in a series of washing stations with the liquid penetrating said mat in each station, removing wash liquid from said mat after each wash station by passing the structure around a rotatable roll whereby liquid is removed from the mat by compressive action and separating the mat from between said belts.

8. The method of claim 7, and including the step of applying fresh wash water to the composite structure at the last wash station of said series, and conducting wash liquid extracted from the mat in each wash station to the preceding upstream wash station.

9. The method of claim 8, wherein the wash liquid is conducted to each preceding upstream wash station by gravity.

10. The method of claim 7, wherein said composite structure is moved in an upwardly inclined path to the horizontal as the wash liquid is applied thereto.

11. The method of claim 7, and including the step of moving the belts in a converging path and said second belt is brought into contact with said mat to thereby squeeze said mat between said belts and extract liquid from said mat.

12. A method of treating a liquid slurry containing fibrous material, comprising the steps of applying a layer of a slurry containing fibrous material to the upper surface of a moving first porous belt to form a mat, bringing a second moving porous belt into contact with the upper surface of said mat to sandwich the mat between said belts and provide a composite belt structure, moving the belts in a converging path to extract liquid from the mat, passing the composite belt structure over a hollow rotatable roll, subjecting the impregnated mat to a pressure differential as it passes over said roll to extract said liquid from said mat, and applying washing liquid to the composite structure as it passes over the roll to impregnate the mat with additional liquid.

13. The method of claim 12, and including the step of applying pressure to the composite structure after the composite structure passes from said roll to extract further liquid from said mat.

14. The method of claim 13, wherein the step of applying pressure comprises passing the composite structure into contact with a pressure plate disposed at a converging angle with respect to the path of travel of said composite structure.

15. The method of claim 12, wherein the step of moving the belts in a converging path comprises supporting the first belt on a series of spaced parallel transverse first rollers, engaging a series of spaced parallel transverse second rollers with the upper surface of said second belt, said second rollers lying in a plane disposed at an acute angle to the plane of said first rollers.

16. The method of claim and and including the step of positioning at least a portion of said second rollers partially within the spaces between the first rollers whereby said composite belt structure moves in an indulating path.

17. The method of claim 12, wherein said hollow roll includes a series of radially extending fins defining a plurality of peripheral compartments, said step of applying a pressure differential comprises applying a vacuum to a group of adjacent compartments to draw liquid from said mat into said compartments.

18. The method of claim 17, wherein the step of applying wash liquid comprises distributing said liquid to the portion of the composite belt structure immediately aftertrayelling over said group of compartments.

19. The method of claim 17, and including the step of separating the second belt from said web immediately prior to applying said washing liquid to said web.

20. The method of claim 12, and including the step of mounting said series of second rollers for pivoting movement in a vertical path, and adjusting the vertical position of said second series of rollers to provide said converging path.

21. The method of claim 7, and including the step of rotating at least one of said parallel rolls at a faster speed than the speed of movement of said first belt to create a vacuum pulse and aid in removal of said liquid from said mat.

22. An apparatus for treating a liquid slurry containing solid material, comprising a headboxto contain a liquid slurry of solid material, means for applying a layer of said slurry to the upper surface of a porous belt, drive means for moving said first belt in an endless path, a second porous belt, second drive means for moving said second belt in an endless path, said second belt being arranged to move in a converging path towards said first belt at a location downstream of said headbox to thereby sandwich the mat between said belts and provide a composite belt structure, means for applying wash liquid to the upper surface of said composite structure with said liquid penetrating said mat, an extracting roll, said composite structure being mounted to travel in a curved path over said extraction roll to extract washing liquid from said mat, and means for separating the mat from said belts.

23. The apparatus of claim 22, wherein said apparatus includes a series of wash stations to supplywash watertothe upper surface of said composite structure, and said apparatus includes an extracting roll located downstream in the direction of travel of said composite structure from each wash station to extract wash liquid from said mat.

24. The apparatus of claim 23, and including means for moving the composite structure in an upwardly inclined path of travel at an angle of 5 to 30 to the horizontal as said liquid is applied to said composite structure.

25. The apparatus of claim 23, in which each wash station includes means to flow the wash liquid onto the upper surface of said composite structure in the form of a puddle, and retaining means disposed along each side edge of the composite structure to prevent said puddle from flowing from the sides of said structure.

26. An apparatus for treating a liquid slurry containing solid material, comprising a first endless belt mounted to travel in an endless path of travel, a second porous filter belt disposed to travel in an endless path of travel, feeding means for applying a layer of a liquid slurry of solid material to the upper surface of said first porous belt to form a mat, said second belt being arranged to move in a converging path towards said first belt at a location downstream of said feeding means to sandwich the mat between said belt and provide a composite belt structure, an initial liquid extraction unit disposed downstream of said converging path for extracting liquid from said mat, a series of wash stations disposed downstream from said initial extraction unit for applying wash liquid to the upper surface of the composite structure, said wash stations being disposed above said feed means and above said initial extraction unit, liquid removal means located downstream of each wash station to remove liquid from the mat, and means located dowstream of said last liquid removal station for separating the mat from the belts.

27. The apparatus of claim 26, and including means for supplying fresh wash water to the final downstream wash station, and means for conducting the wash liquid removed at each removal station to the next succeeding upstream wash station.

28. The apparatus of claim 26, wherein each succeeding wash station located in a downstream direction is positioned at a higher level than the preceding wash station whereby the wash liquid will flow from gravity to said next upstream wash station.

29. The apparatus of claim 26, wherein said separating means is located at the same end of the apparatus as said feeding means.

30. The apparatus of claim 26, wherein said initial extraction unit includes a plurality of generally parallel rotatable rolls disposed in a curved pattern, said composite structure disposed to travel over said rolls.

31. The apparatus of claim 30, wherein the curvature of said pattern is greater at the downstream end of said extraction unit.

32. The apparatus of claim 30, and including a generally vertical deflecting plate disposed between adjacent first rolls.