WO1990010750A1

WO1990010750A1 - Method and apparatus for washing cellulosic pulp

Info

Publication number: WO1990010750A1
Application number: PCT/US1990/001319
Authority: WO
Inventors: Norbert Kessler
Original assignee: Green Bay Packaging, Inc.
Priority date: 1989-03-14
Filing date: 1990-03-08
Publication date: 1990-09-20
Also published as: CA2028823A1; FI905601A0

Abstract

A pulp washer for washing cellulosic pulp stock. The pulp washer includes an endless porous moving belt (33) that rides on a perforated rigid support plate (17) and the cellulosic stock is fed as a layer or mat (69) onto the upper surface of the belt (33). The mat is progressively reduced in thickness and liquor is extracted from the mat as it moves through the converging path. The mat is then passed through a series of conditioning or washing stations where the mat is subjected to dilution by washing and drainage cycles. Each washing station includes a standpipe (103) that extends upwardly beyond the belt and contains a wash liquid. The upper surface of liquid in the standpipe is exposed to a gas (106), such as air, so that the pressure head of the liquid in the standpipe will drive the liquid through the mat.

Description

"Method and Apparatus For Washing Cellulosic Pulp"

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 a substantial amount 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 suc¬ tion 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 countercurrently to 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 cntrol the atmosphere in the hood and maintain the desired pres¬ sure 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 substan¬ tially greater no er requirement and also causes wear of the filter belt, reducing the service life of the belt.

U.S. patents 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 con¬ tained 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 mat 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 con¬ taining solid materials, such as a slurry composed of cellulosic pulp stock and digester or pulping liquor. The flowable cellulosic slurry is fed as a layer or mat onto the upper surface of a moving endless porous belt that rides on a rigid perforated polymeric support plate. An upper inclined plate is spaced from the belt and defines with the belt a converging flow path so that the mat is progressively reduced in thickness as it passes through the converging flow path and liquor is extracted from the mat.

The mat then passes through a series of con¬ ditioning or washing stations where the pulp mat is subjected to dilution by washing liquid and drainage cycles. Fresh water is delivered to the mat at the final downsteam station and the washing liquid, after passing through the mat, is pumped in countercurrent flow through the various conditioning stations to the upstream end of the washer.

Each washing or conditioning station comprises a standpipe which extends upwardly beyond the belt and the upper surface of the liquid in the stand- pipe is exposed to a gas, such as air, so that the pressure head of the liquid in the standpipe will drive the liquid through the pulp mat. The wash liquid draining through the mat is collected in a sump and is then pumped to the standpipe associated with the next upstream washing station, so that in this manner, the wash liquid is circulated in a countercurrent flow with respect to the flow of the pulp mat through the washer. Following the final washing or conditioning station, the mat is separated from the endless belt.

As the wash liquid is driven through the mat by hydraulic pressure, without vacuum or forced air flow, foaming is minimized. Further, the air cushion in the standpipe provides improved control of the hydraulic pressure in each washing or conditioning station.

The washer is a completely enclosed unit, so that there is no discharge of pollutants into the atmosphere. The washer comprises a lower housing sec¬ tion and an upper housing section, which is hinged to the lower section along a side edge and the two housing sections define a sealed washing or conditioning cham¬ ber. The upper housing section can be pivoted upwardly to an open position to install the belt or for mainten¬ ance or repair. The headbox is an integral part of the upper housing section.

In addition, a seal is provided at the up¬ stream end of the conditioning chamber which seals against the belt as it enters the conditioning chamber to preclude discharge of pollutants into the atmos¬ phere. Similarly, a second seal is provided at the downstream end of the conditioning chamber which seals against the upper surface of the pulp mat, as the mat is discharged from the conditioning chamber.

Any air entrapped in the mat will bubble upwardly through the wash liquid contained within the conditioning chamber and be collected in the head space in the standpipes. To maintain the desired air cushion in the headspace, a relief valve en be incorporated with the upper end of each standpipe and a liquid level control, which senses the liquid level in the standpipe can be incorporated to maintain the proper liquid level in each standpipe.

The pulp washer of the invention provides a low dilution factor and will condition the pulp with a low soda loss as well as a low fiber loss.

The pulp washer is compact in size and occupies considerably less floor space than pulp washers, as used in the past.

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:

Fig. 1 is an end view of the pulp washer of the invention;

Fig. 2 is a side elevation of the pulp washer of the invention;

Fig. 3 is a fragmentary enlarged longitudinal section of the upstream end of the washer;

Fig. 4 is a fragmentary enlarged longitudinal section of the central portion of the washer;

Fig. 5 is a fragmentary longitudinal section of the downstream end of the washer;

Fig. 6 is a transverse section of the washer; and

Fig. 7 is an enlarged fragmentary longitud¬ inal section showing the seal at the upstream end of the washer. Description of the Illustrated Embodiment The drawings illustrate a pulp washer composed of a frame 1 that supports a closed housing 2. Frame 1 includes a pair of upper horizontal beams 3 each of which is spaced above a lower horizontal beam 4. As shown in Fig. 1, the ends of each pair of beams 3 and 4 are connected by a threaded screw jack 6, while the opposite end of each lower beam 4 is provided with an upwardly extending extension 7 and a removable block 8 is disposed between the extension 7 and the lower surface of the end of the corresponding upper beam 3. By removal of the block 8, the lower run of the endless mat supporting belt can be inserted in the frame, as will be hereinafter described.

Housing 2 is composed of a generally trough- shaped lower section 10 and an upper section 11 which is hinged to the lower section along the corresponding side edges.

As illustrated in Fig. 6, lower section 10 of housing 2 includes a pair of spaced side channels 12, the ends of which are connected by end plates 13. In addition, a bottom wall 14 is connected to the side channels 12 and end plates 13 to provide the trough¬ like construction for the lower housing section 10. As shown in Fig. 6 , an inverted channel 15 extends inwardly from one of the side channels 12 and a longi¬ tudinal beam 16 is secured to the inner upstanding flange of channel 15. A similar beam 16 is secured to the web of the opposite side channel 12. As shown in Fig. 6, a rigid plate or block 17 is supported on the upper edges of beams 16 and extends the length of the housing 2. The plate 17 is preferably formed of a polymeric material, such as polyethylene, and contains a plurality of parallel, elongated slots 18 which extend transversely of the plate. Plate 17 serves to support a porous belt which, in turn, supports a web or mat of pulp, as will be hereinafter described.

Lower section 10 of housing 2 is provided with a plurality of spaced transverse divider walls 19, which divide the lower housing section 10 into a series of sumps or chambers 20 that are spaced along the length of the housing. As shown in Fig. 2, six sumps 20a-f are provided in lower section 10.

A vertically adjustable wall member 21 is provided at each end of lower section 10. The wall member 21 is vertically adjustable with respect to divider wall 19 through a bolt and slotted connection 22, and the upper edge of wall member 21 is provided with an inwardly bent tip or flange 23, which is engaged with a groove in the respective end of plate 17. Engagement of edge 23 with the groove in plate 17 acts to hold the ends of the plate down and accommod¬ ates differential heat expansion between the polymeric plate 17 and the metal lower housing section 10.

The walls 19, along with the end walls 13, define chambers 24 and 25 at the upstream and down¬ stream ends, respectively, of lower section 10.

Upper housing section 11 is composed of a pair of spaced side channels 27 which are connected by a top wall or cover 28. As shown in Fig. 6, a side channel 29 is secured to the lower portion of the web of each channel 27 and the side flanges of each channel 29 are welded to a plate 30. Each plate 30 extends outwardly from channel 29 and the outer portion of plate 30 is connected by bolts to the lower flange of the respective side channel 27. Each channel 29 is provided with a plurality of vertical tubes 31 which are spaced along the length of the channel. Tubes 31 extend completely through the channels and provide passages for the flow of the wash liquid, as will be hereinafter described. As best illustrated in Fig. 6, the lower surface of each plate 30 is provided with a recess 32 and the side edges of a porous endless belt 33 are received within the respective recesses. The upper run of belt 33 is supported on the upper surface of plate 17 and the side edges of the belt are spaced inwardly of the tubes 31. With this construction, wash liquid flowing downwardly through tubes 31 will then flow inwardly through recesses 32 to prevent any fibrous material from clogging within the recesses.

The abutting side edges of the housing sec¬ tions 10 and 11 are sealed by gaskets 34. One of the gaskets 34 is carried on the upper flange of a channel 35 that is mounted on the upper flange of side channel 12, while the other gasket 34 is carried by the upper end of a generally L-shaped bracket 36 that extends upwardly from channel 15. When the upper housing sec¬ tion 11 is in the closed position, the lower surfaces of the respective plates 30 will bear against the gaskets 34 to provide a seal along the side edges of the housing sections.

Upper housing section 11 is adapted to be pivoted between a closed and open position. To provide the pivoting action, a group of pivot blocks 37 are mounted on the upper flange of one of the side channels 12 of lower section 10, while a corresponding group of lugs 38 are secured to one of the side channels 27 of upper section 11, and the lugs 38 are pivoted to blocks 37 by pivot shaft 39. At the opposite side of the housing, as illustrated in Fig. 6, the lower end of a screw jack 40 is secured to a pivot block 41 which is pivotally connected through pin 42 to lug 43 secured to side channel 12 of the lower section 10. Jack 40 can be swung upwardly and received within a slot in the flange 46 of bracket 47 that is secured to the corres- ponding side channel 27 of upper section 11. A pair of nuts 48 are threaded on jack 40 and located beneath flange 46, while a second pair of retaining nuts 49 are engaged with the upper end of the jack and bear against the upper surface of flange 46. Engagement of nuts 48 and 49 with jack 40 will lock upper section 11 in the closed position. By releasing nuts 49 and threading nuts 48 on the jack, the upper housing section 11 can be pivoted about shaft 39 relative to the lower section. In addition, the jacks 40 can be swung out¬ wardly and downwardly to a position where they will not interfere with installation of the belt 33 in the housing.

Belt 33 is carried by a drive roller 51 located at the downstream end of the housing and an idler roller 52 located at the upstream end. In travelling from roller 52 to roller 51, the upper run of porous belt 33 is supported on the upper surface of plate 17. Belt 33 can be formed of a plastic material, such as nylon, or can be formed of a metal, such as stainless steel.

Rollers 51 and 52 are each supported in a pair of bearing blocks 53 which are secured to the side channels 12 of lower section 10 through support members 54. As shown in Fig. 2, the lower run of belt 33 is trained over a roller 55 which is located intermediate the length of the housing and the roller 55 is support¬ ed from the lower section 10 through supports 56. As previously described, the lower run of belt 3 can be installed by removal of the blocks 8, while the upper run of the belt is installed by releasing the jacks 40 and pivoting the jacks downwardly, so that the upper run of the belt can be inserted between the upper and lower housing sections. Mounted on the upper edge of the end wall 13 at the upstream end of housing section 10 is a guide block 57 which supports the belt 33. As shown in Fig. 3, guide block 57 is spaced between roller 52 and the upstream end of plate 17. Block 57 can be formed of a polymeric material, such as nylon. Similarly, a guide block 58 is mounted on the upper edge of end wall 13 at the downstream end of the lower section and serves to guide the belt 33 as the belt travels from the down¬ stream end of plate 17 to drive roller 51.

The cellulosic slurry is fed to the upstream end of upper housing section 11 through a supply conduit 60. As best shown in Fig. 1, supply conduit 60 includes a vertical pipe 61 which is connected through a flexible coupling 62 to elbow 63. Elbow 63, in turn, is connected to a horizontal manifold 64 which extends transversely of the housing 1 and a group of inlet pipes 65 connect the manifold 64 with the enlarged inlet 66 of upper housing section 11. See Fig. 3. Inlet section 66 defines an inlet chamber 67 which extends both upwardly and downwardly from the inlet pipes 65. This results in a swirling action for the incoming slurry which aids in providing a homogenous mixture of fibers and liquid.

The incoming slurry is carried by belt 33 into a converging flow path defined by plate 17 and an inclined mat-confining member 68, which extends across the width of the housing. As the slurry passes through the converging path, liquid is extracted from the slurry to provide a fibrous mat 69 and the liquid flows through the slots in plate 17 into the upstream sump 20a. The mat-confining member 68 includes a flat bottom plate 70 which is disposed at an acute angle with respect to plate 17 and a group of generally L- shaped reinforcing members 71 are secured to the upper surface of plate 70, while a generally vertical plate 72 extends upwardly from the upstream end of plate 70. The downstream end of plate 70 is secured by bolts to a pivot strip 73 which extends transversely across the upper housing. As the bottom plate 70 is somewhat flexible, it can be pivoted in a vertical direction about the connection with strip 73.

To lock the mat-confining member 68 at the desired inclination, a group of lugs 74 are connected between the vertical section 72 and the downstream reinforcing member 71 and each lug is provided with an elongated slot 75. Lugs 74 are disposed flatwise against the side channels 27 of the upper section and bolts 76 extend through the slots 75, as well as through openings in the channels 27. With this con¬ struction, the mat-confining member 68 can be pivoted in a vertical plane relative to plate 17 to change or vary the angularity therebetween and the member 68 can be locked into the desired position by turning down bolts 76.

To seal the upstream end between housing sections 10 and 11, a sealing unit 77 is employed. Sealing unit 77, as best illustrated in Fig. 7, includes an elongated block 78 which extends trans¬ versely of the belt 33. The upstream edge of block 78 is provided with a recess and the lower end of end wall

79 of upper section 11 is secured in the recess by welding or the like. A flexible wiper 80 is mounted within a recess in the downstream end of block 78 by strip 81, and the outer generally flat end 82 of wiper

80 rides against the upper surface of belt 33 to provide a seal at the upstream end of the housing. Any liquid leakage which should occur between wiper 81 and belt 33 is collected in the upstream chamber 24. A second mat-confining member 85 is located downstream of the member 68 and forms an extension thereto, as shown in Fig. 3. Mat-confining member 85 includes a lower generally flat plate 86 which extends across the housing and is located at an acute angle with respect to the support plate 17, to thereby define a converging flow path for the mat 69. The converging flow path aids in compressing and extracting liquid from the mat. A reinforcing beam 87 is secured to the upper surface of plate 86 and the upstream end of plate 86 is secured to a pivot strip 88 that extends trans¬ versely across the upper housing section. The flexib¬ ility of plate 86 enables the plate to be tilted or pivoted in a vertical plane relative to the strip 88.

To pivot plate 86, an arm 89 is connected to a cross shaft 90 that extends transversely across the upper housing section 11, and a link 91 interconnects the outer end of arm 89 with the downstream end of the reinforcing member 87.

A generally vertical arm 92 is also secured to shaft 90 and the upper end of arm 92 is connected through two pair of inflatable air bags 93 and 94 to fixed supports 95. As shown in Fig. 3, the air bags 93 and 94 are located on opposite sides of the arm 92 and by inflating or deflating the bags, the arm 92 can be pivoted to thereby pivot link 89 and pivot the mat- confining member 85. For example, by inflating the air bags 93, member 85 will be raised, and conversely by inflating bags 94, member 85 will be lowered. Adjust¬ able stops 96 and 97 are located on opposite sides of the arm 92 and are each mounted through a fixed bracket 98 to the housing. Stops 96 and 97 limit the pivotal movement of the arm 92 and thus limit the pivotal move¬ ment of mat-confining member 85. Upper housing section 11 includs a vertically enlarged section 100 spaced above member 85 and defin¬ ing a closed liquid chamber 101. The upstream end of section 100 terminates adjacent the upstream end of member 85. The pressure of the liquid within the chamber 101 defined by section 100 aids in resisting the upward pressure exerted by the mat 69 against member 85 as the mat flows through the converging flow path.

A series of wash or conditioning zones are located along the length of housing 2 and, as shown in the drawings, five such zones are provided. A stand- pipe 103a-e is associated with each zone and acts to supply wash liquid to that zone. Each standpipe 103 includes a vertical column 104, the lower end of which communicates with the liquid chamber 101, while the upper end of column 104 is in communication with an enlarged generally spherical head 105. Air, or other gas, is located in the headspace 106 above the liquid level in head 105.

The lower end of column 104 of each standpipe 103 terminates in an inclined surface 107 which is positioned at an acute angle to the horizontal and aids in directing the wash liquid from the standpipe in a downstream direction within the chamber 101.

The lower end of column 104 of each standpipe 103 is provided with a horizontal flange 108 that is spaced from the inclined surfce 107, as shown in Figs. 3 and 4, and a wiper 109 is associated with the stand- pipes 103b-e. Each wiper 109, as illustrated in Fig. 4, has a generally curved outer edge which rides against the upper surface of mat 69, while the opposite end of each wiper is connected to the flange 108. Wiper 109, wich exends across the width of the housing section 11, can flex or pivot relative to flange 108 and will ride or float on the upper surface of mat 69 to prevent wash liquid from the associated standpipe 103 from flowing in an upstream direction.

A conventional relief valve 111 is associated with the upper end of the head 105 of each standpipe and serves to relieve the pressure in headspace 106, if it exceeds a predetermined value. During operation of the pulp washer, any air entrained in the mat 69 will be vented upwardly during the washing operation and will collect in the headspace 106 of standpipes 103. If the pressure in the headspace is increased beyond a predetermined value, the relief valve will open to vent the pressure.

To seal the area where the mat 69 is dis¬ charged from the housing, a sealing assembly, indicated generally by 114, is incorporated. Sealing assembly 114, as shown in Fig. 5, includes a flexible plate 115 which is disposed at an acute angle with respect to plate 17 and the outer or downstream edge of plate 115 rides on the upper surface of mat 69. A box-like re¬ inforcing member 116 is secured to the upper surface of plate 115 and extends the width of the plate. The upstream end of plate 115 is connected to a pivot strip 117, which extends transversely of the upper housing section 11. Due to the flexible nature of plate 115, the plate can pivot or flex in a vertical direction about the pivot strip 117.

To pivot plate 115, a pivot shaft 118 extends transversely across upper housing section 111, and a bell crank lever 119 is secured to shaft 118. The lower, generally horizontal arm 120 of lever 119 is connected via a link 121 to the downstream end of re¬ inforcing member 116, while the vertical arm 122 of lever 119 extends upwardly from shaft 118. Inflatable air bags 123 and 124 are mounted between the arras 120 and 122 of lever 119 and a fixed angle bracket 125. By inflating the air bags 123, arm 120 will be pivoted downwardly to thereby pivot plate 115 downwardly toward plate 117. Conversely, inflating bags 124 will pivot arm 120 clockwise as shown in Fig. 5, thereby tilting the plate 115 upwardly to increase the spacing between the plate and the support plate 117.

To limit the pivotal movement of the lever 119, a lug 126 is secured to the upper end of arm 122 and is adapted to engage nuts 127 and 128 which are threaded on rod 129 that is carried by fixed bracket 130 attached to angle 125. Engagement of lug 126 with nut 127 will limit the upward movement of plate 115, and conversely, engagement of lug 126 with nut 128 will limit the downward movement of plate 115.

As plate 115 of sealing assembly 114 rides on the upper surface of the compressed mat 69, it will effectively seal the downstream end of the housing to the ingress of air.

Any wash liquid leaking past the sealing assembly 114 will be collected in chamber 125 that is located at the downstream end of the housing. As pre¬ viously noted, any wash liquid leaking through the sealing assembly at the upstream end of the housing 2 will be collected in chamber 24. In addition, a pro¬ vision is made to collect any wash liquid leaking between the support plate 17 and plates 30 at the sides of the housing. Any leakage at this area will be col¬ lected in the side chambers 132 and 133, respectively, at the sides of the housing, as shown in Fig. 6. Con¬ duits 134, 135, 136 and 137 are connected to the lower end of the respective chambers 24, 25, 132 and 133, and the wash liquid is conducted through the conduits to an accumulation tank, not shown. The wash liquid thus collected can then be returned to the upsteam end of the liquid chamber 101 through a return conduit 138, which is mounted within an opening in the section 100, as shown in Fig. 3.

The wash liquid is preferably fed counter- currently through the pulp washer with respect to the flow of the mat 69. In this regard, a conduit 140 interconnects the lower end of each sump 20c-f with the inlet of a pump 141, while the outlet of the pump is connected via conduit 142 to the standpipe 103 of the next upstream zone, so that the wash liquid from each zone will be delivered to the next upstream zone in a countercurrent fashion. Fresh wash liquid is introduced into the downstream standpipe 103e through an inlet conduit 143 and the spent wash liquid is dis¬ charged from the sumps 20a and 20b through outlet con¬ duits 144, which are connected through pumps 145 to a disposal site. With this arrangement, fresh wash liquid is introduced to the standpipe 103e through inlet conduit 143 and the wash liquid will then pass through the mat and be collected in sump 2Of. The liquid will then flow from sump 20f through conduit 140 to pump 141f and will be discharged through conduit 142 to the next upstream standpipe 103d. This procedure is repeated with the wash liquid progressing countercur¬ rently through the system and finally being discharged through the conduits 144 to a disposal site.

It is contemplated that a liquid level sensing mechanism can be incorporated with the stand- pipes 103 to maintain the desired liquid level in the head 105, as shown in Fig. 4. A pair of liquid level sensing probes 145 and 146 are disposed in each head 105 and are operably connected to a valve 107 located in the discharge line 142 that supplies that standpipe. If the liquid level in the head 105 falls below a given value, as sensed by probe 145, the valve will be opened to admit liquid through conduit 142 to the standpipe. On the other hand, if the liquid level rises above the level of probe 146, valve 147 will close to prevent the supply of liquid to the standpipe. With this arrange¬ ment, the liquid level is maintained within predeter¬ mined limits within the standpipe to provide the desired pressure head for driving the liquid through the mat 69.

In operation, the cellulosic slurry is fed into the inlet chamber 68 through the feed conduit 60 and the configuration of the inlet chamber promotes swirling of the slurry to intimately mix the fibrous material and liquid. The slurry then flows downstream into the converging flow path between plate 17 and the mat-confining members 68 and 85. As the slurry flows in the converging path, liquid is extracted from the slurry and flows through the porous belt and through the slots 18 in plate 17 for collection in the sump 20a. As previously noted, the pressure of the wash liquid in section 100 above member 85 resists the up¬ ward pressure of the mat flowing through the converging flow path to prevent upward deflection of member 85.

As the mat enters the liquid chamber 101, the wash liquid is driven through the fibrous mat because of the pressure head generated through use of the standpipe 103. The air cushion in the upper head 105 of the standpipe provides improved control of the hyd¬ raulic pressure in each wash zone.

As the mat is conveyed on belt 33 through the washer, the wash liquid is driven through the mat in the various washing zones to provide a thoroughly washed mat which is discharged from the belt as the belt passes around the roller 51 at the downstream end of the washer. The pulp washer of the invention has a low dilution factor and has low soda and fiber loss.

As the housing is a closed system there is discharge of pollutants into the atmosphere. Due to its compact size, there is a substantial space savings as compared to conventional pulp washers.

Since the wash liquid is driven through the mat by hydraulic pressure without the use of vacuum or air flow, foaming of the liquid is minimized.

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

Claims

We claim:

1. An apparatus for treating a liquid slurry containing solid material, comprising a supporting frame, a perforated support member, means for mounting the support member in a conveying run, drive means for moving said support member in said conveying run, feed¬ ing means for feeding a slurry containing solid material onto said conveying run of said support member to form a layer, means disposed downstream of said feeding means and defining a converging flow path with said conveying run whereby said layer is progressively reduced in thickness as it passes through said converg¬ ing flow path, and supply means disposed downstream of said converging flow path for supplying a wash liquid to the upper surface of said layer, said supply means including a vessel to contain said liquid and extending a substantial distance above said conveying run, the upper surface of said liquid in said vessel being exposed to a gas whereby the head of said liquid will drive said wash liquid through said layer.

2. The apparatus of claim 1 and including a closed housing to enclose said support member and the layer supported thereon.

3. The apparatus of claim 2 wherein said housing comprises a lower section and an upper section, means for pivotally connecting the side edges of said sections together, and means for pivoting one of said sections relative to the other of said sections to thereby open said housing.

4. The apparatus of claim 3, wherein said means for pivoting one of said sections comprises at least one screw jack.

5. The apparatus of claim 1, and including a fixed perforated support to support the conveying run of said support member.

6. The apparatus of claim 5, wherein said perforated support comprises a block of polymeric material containing a plurality of openings.

7. The apparatus of claim 6, wherein said block is composed of polyethylene and said openings comprise a plurality of slits extending transversely of the direction of travel of said support member.

8. The apparatus of claim 1, wherein said means for defining a converging flow path comprises a plate disposed at an acute angle with respect to said support member, and means for biasing said plate toward said support member.

9. The apparatus of claim 8, and including means for pivoting the upstream end of said plate to said frame.

10. The apparatus of claim 9, wherein said biasing means comprises an inflatable air bag.

11. The apparatus of claim 2, and including sealing means at the downstream end of the conveying run and disposed to engage the upper surface of said layer to prevent ingress of air into said housing.

12. The apparatus of claim 11, wherein said sealing means comprises a sealing member extending across the width of said support member, pivot means for pivoting the upstream end of said sealing member to said frame, and biasing means operatively engaged with said sealing member to bias the sealing member toward said conveying run.

13. The apparatus of claim 12, wherein said sealing member is disposed at a converging angle with respect to said conveying run.

14. The apparatus of claim 1, and including a sump located beneath said support member to collect wash liquid passing through said support member.

15. The apparatus of claim 2, wherein said conveying run includes a plurality of zones spaced along the length of said conveying run, said apparatus including a vessel associated with each zone, a sump associated with each zone and disposed beneath said conveying run to collect liquid passing through said support member, and pumping means for each sump, each pumping means being operably connected to a vessel of a different zone whereby operation of said pumping means with deliver wash liquid from the sump to said vessel.

16. The apparatus of claim 14 wherein each pumping means is connected to a vessel of an adjacent upstream zone.

17. An apparatus for treating a liquid slurry containing a solid material, comprising a closed housing, an endless perforated belt disposed within the housing and mounted for travel in an upper conveying run, drive means for driving said belt in said run, feeding means for feeding a liquid slurry containing solid material to the upstream end of said conveying run to form a layer, rigid perforated support means for supporting said conveying run of said belt, and supply means for supplying a wash liquid to the upper surface of said layer and including a vessel to contain said liquid and extending upwardly a substantial distance above said conveying run, the upper surface of said liquid in said vessel being exposed to a gas whereby the pressure head of said liquid in said vessel will drive the wash liquid through said layer, and collect- ing means for collecting the wash liquid passing through said layer.

18. The apparatus of claim 17, wherein said housing comprises a lower section and an upper section, means for pivotally connecting the side edges of said sections together, and means for pivoting one of said sections relative to the other of said sections to open said housing.

19. The apparatus of claim 18, and including sealing means located at the downstream end of said conveying run and disposed to engage the upper surface of said layer to prevent ingress of air into said hous¬ ing.

20. The apparatus of claim 18, wherein said upper and lower sections have mating side edges, and gasket means disposed between said side edges to seal said side edges against the ingress of air.

21. The apparatus of claim 18, wherein said means to pivot one of said sections comprises a threaded jack pivotally connected to said lower section and extending through an opening in said upper section, and nut means threaded on said jack and located beneath said upper section whereby threading of said nut on said jack will pivot said upper section relative to said lower section.

22. An apparatus for treating a liquid slurry containing solid material, comprising a closed housing, an endless perforated belt mounted for travel in said housing in a conveying run, drive means for driving said belt in said conveying run, feed means disposed at the upstream end of said conveying run for feeding a liquid slurry containing solid material onto said conveying run, means disposed at the downstream end of said conveying run for removing said layer from said belt, a series of wash zones spaced along the length of said conveying run, a sump associated with each zone and located beneath said conveying run to collect liquid passing through said belt, a vessel associated with each zone and extending upwardly a substantial distance above said conveying run, the upper surface of liquid in said vessel being exposed to a gas whereby the head of liquid in the vessel will drive said liquid through said layer, and pumping means associated with each zone for pumping liquid from the sump of that zone to a vessel of a different zone.

23. The apparatus of claim 22, wherein each pumping means is connected to a vessel of an adjacent upstream zone.

24. The apparatus of claim 22, wherein each vessel comprises a vertical conduit and an enlarged head disposed at the upper end of each conduit.

25. The apparatus of claim 24, wherein said head is exposed to the atmosphere.

26. The apparatus of claim 1, and including liquid level sensing means disposed in said vessel to sense the level of liquid therein, said sensing means being operably connected to said supply means and con¬ structed and arranged to supply wash liquid to said vessel when the level of liquid in said vessel decreases below a predetermined level.

27. The apparatus of claim 22, and including liquid level sensing means disposed in each vessel and disposed to sense the level of liquid in said vessel, each sensing means being operably connected to a pump¬ ing means of a different zone to supply wash liquid from said different zone to said vessel when the liquid level in the vessel recedes beneath a predetermined level.

28. An apparatus for treating a liquid slurry containing solid material, comprising a support- ing frame, a perforated support member, means for mounting the support member in a conveying run, drive means for moving said support member in said conveying run, feeding means for feeding a slurry containing solid material onto said conveying run of said support member to form a layer, a housing located above said support member and defining a liquid chamber, converg¬ ing means disposed downstream of said feeding means and defining a converging flow path with said conveying run whereby said layer is progressively reduced in thick¬ ness as it passes through said converging flow path, said chamber extending above said converging means, biasing means operably connected to said converging means for biasing said converging means toward said support member, and supply means for supplying a wash liquid to said chamber, said wash liquid passing through said layer, the portion of said wash liquid in said chamber and disposed above said converging means resisting upward deflection of said converging means due to the pressure of said layer.

29. The apparatus of claim 28, wherein said supply means comprises a generally vertical standpipe extending a substantial distance above said chamber and containing said wash liquid, the pressure head of said liquid in said standpipe driving said liquid through said layer.

30. The apparatus of claim 29, and including means for maintaining a substantially constant pressure head in said standpipe.

31. The apparatus of claim 29, wherein said standpipe is constructed and arranged to exert a pres¬ sure head of sufficient magnitude to drive said liquid by gravity through said layer.