US3595038A - Apparatus for washing fibrous suspensions - Google Patents

Abstract

In washing paper pulp or similar fibrous suspension the suspension is moved intermittently vertically through a column. In the dwell periods, washing liquid is forced through the fibrous suspension, and the resulting ''''extract'''' removed, in a generally horizontal direction through perforate screening members forming portions of the column.

Description

United States Patent [56] References Cited UNITED STATES PATENTS 10/1890 Forbes.......................... 1,864,220 6/1932 172] Inventors Carl Arne Bergholm Per-Erik Andersson, Sundsvell; Gustaf 111 011 0 6l/l/6 ma ma 6 I16 1 1 X l 5 n 2 6 Rune llellerqvist, Sundsvall, all of, Sweden 439,033 [2]] Appl. No. 737,161 [22] Filed June 14, 1968 3,078,703 2/1963 R chter et a1. 45 p m d July 37 197 3,088,306 5/1963 Rrchter.......... 73 Assignee Svenska Cellulose Aktiebolaget 3,372,087 3/1968 R chter... sund v ll, Sw d 3,385,753 5/1968 RlCh [32] P i ig June 10, 1967 3,425,898 2/1969 Laakso Sweden 3,475,271 10/1969 Laakso............

FOREIGN PATENTS 150,782 9/1920 Great Britain......

Primary Examiner-Danie1 Blum ABSTRACT: In washing paper pulp or similar fibrous suspeny vertical1y through ng liquid is forced resulting extract gh perforate r eumc 0 mon n n .mo

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APPARATUS FOR WASHING FIBROUS SUSPENSIONS In the manufacture of pulp from vegetable materials by cooking and when bleaching or otherwise treating pulp, the treated pulp is often obtained in the form of a suspension of fibers in an aqueous solution containing greater or smaller amounts of reaction products of organic or inorganic nature. Before this fibrous suspension can be subjected to further working or treatment a great part of these reaction products has to be removed by washing with water or with aqueous solutions having a substantially lower content of reaction products than that of the original solution. Furthermore it is desirablethat this washing be carried out as completely as possible by using as small amounts of water as possible.

Heretofore the washing has been carried out in discontinuous diffusers, for example in sulfate digesters or bins, or in sulfite digesters, or in filter washing plants which are in general use in bleaching departments in connection with washing operations between the various bleaching steps and also for washing and recovering waste liquors from sulfite and sulfate digestion.

The discontin'uous diffusers, the washing in digesters and the washing in bins are characterized in that the pulp being washed is strongly compressed and rionhomogeneous. Usually the flow throughthe pulp suspension becomes very irregular, and channeling often occurs, due to which there is a poor washing effect. For improving this, it has been necessary to wash in series, displacing the original cooking liquor by weak, or diluted liquor, and displacing, in its turn, the diluted liquor after the first washing step by water, and then reserving the displaced weak liquor for the next washing operation. At times it has been necessary to wash with several weak liquors of different concentrations, for the purpose of obtaining an acceptable washing without the recovered extract being too much diluted with water. This type of procedure necessitates the handling of considerable amounts of weak liquor and requires the use of large storage tanks. Furthermore, these operations require a substantial amount of manual work or an extensive equipment.

In filter washers the pulp suspension is diluted, with recovered extract, from an original consistency of 4 l 5 percent to about 1 percent consistency in order to put the suspension in a condition for being collected on the filter. The resulting fiber web then is washed with weak liquor or water which is sprayed upon the web. The effectiveness of the washing carried out on the filter is limited by the pulp web being uneven and compressed, and furthermore by the difficulty of uniformly distributing the washing liquor on the web by spraying. For obtaining a fully satisfactory washing result, it often is necessary to connect several filters in series. Such a washing plant is, of course, very expensive and it requires large investments in filters, backwater tanks, pumps and the like.

The present invention resides in a new method of washing, in a column or in some other vessel, a product, particularly chemical pulp or other fibrous material, which is suspended or otherwise distributed in a liquid and is characterized in that the fibrous suspension is caused to move intermittently within the column in such a manner that it will successively through a washing zorie of said column, and that during the periods of time in which the fibrous suspension is not being moved, the liquid of the fibrous suspension in said washing zone is dis placed and separated as an extract by means of washing liquid, the direction of flow of which intersects the overall direction of movement of the fibrous suspension in the column. The washing zone can occupy the entire length of the column, or only a part thereof, in the direction of flow of the pulp suspension.

The fibrous suspension preferably is transported along juxtaposed screening means extending parallel to the flow direction of the pulp, so that during the periods, in which no movement is taking place, the pulp is washed with washing liquid which is caused to flow from a screen through the layer of pulp to an opposite screen in a direction which is substantially perpendicular to the direction of movement of the pulp and at such a low rate that the pressure drop above the layer of pulp is less than the pressure drop required for substantially compressing or displacing the fibrous material between the screens.

At the outlet of the extract screen this method yields an extract having a concentration which closely corresponds to the concentration of the liquid of the supplied unwashed pulp suspension, and at the same time the pulp suspension after the washing operation contains an insignificant amount of the liquid present in the unwashed pulp suspension.

The invention is based on the observation that cellulosic fibers, suspended in a liquid, for example, water at consistencies above approx. 1 percent and stored in a vessel, exert a certain pressure on the surrounding walls of the vessel. This so-called fiber pressure is a function of the consistency and increases with increasing consistency. This fiber pressure also depends on the geometry and the elasticity properties of the pulp, or to be more exact, to each individual pulp fiber. Thus, for example, a long-fibered pulp having a larger fiber diameter will, at a certain consistency, present a greater fiber pressure than a short-fibered pulp having a small fiber diameter will in the same consistency.

The invention further is based on another observation, viz that the fiber network, constituting the pulp suspension, presents a certain inner strength, so that a certain effort or forces is required for changing the shape ofthis fiber network.

These conditions render it possible to construct a vessel for pulp suspensions, in which the fiber network, due to frictional forces, is supported, entirely or partially, by the surrounding walls of the vessel and is not compressed merely by its own weight.

lt also is known, that the fiber network in a pulp suspension is permeable to water, so that the liquid in the suspension, if subjected to a liquid pressure gradient, will flow through the fiber network. By investigations it has been found that, if this pressure gradient applied above a certain layer of pulp is less than ten times the fiber pressure of the fiber network, the liquid will flow through the fiber network without then substantially compressing the same. By investigations the speed of flow of the liquid corresponding to this pressure gradient was found to be very low.

When washing a fibrous suspension by displacing the liquid in the fibrous suspension with another liquid, it is, according to the invention, important that the fiber network is not compressed to any substantial extent. Under such conditions the pulp is loose, and the washing liquid displaces, uniformly and substantially completely, the liquid present in the fiber suspension. Furthermore, under these conditions the excess amount of washing liquid required for the displacement is small compared to the amount of liquid originally present in the fibrous suspension.

It is also known that at a flow through permeable layers the speed of flow is inversely proportionall to the thickness of the layer when the liquid pressure gradient is constant.

As the liquid pressure gradient applied to the washing of a fibrous suspension is small under the conditions ideal for the displacement, when no substantial compression is taking place, and in view of the fact that the speed of displacement is inversely proportional to the thickness of layer, said thickness must be held as small as possible in order that the time of displacement and, hence, the dimensions of the apparatus shall not become too great. According to the invention this has been achieved by arranging the screening surfaces of the washing apparatus parallel with the direction of transportation of the fibrous suspension and close to each other, and so that the displacement takes place substantially perpendicularly to the direction of movement of the pulp from an inlet screen to the opposed extract outlet screen.

Investigations have shown that the layer should not be exceedingly thin, because too thin pulp layers result in an inhomogeneous layer, which yield an inferior displacement.

This minimum thickness of the layer has proved to have a certain connection with the geometry of the pulp fibers, more precisely with the weighed average length of the fibers. Thus, the thickness of the layer must not go below a value corresponding to about twenty times the average fiber length.

The upper limit for the thickness of the pulp layer is determined by the practical obstacles caused by excessive draining time with resulting large apparatus dimension. Thus, the thickness of the pulp layer should not exceed about 1 meter.

Part of the liquid in a pulp suspension is not available for direct displacement during a rapid washing, because it is enclosed in the fiber. Due to the fact, however, that the washing method according to the invention provides for washing times of at least one minute, or longer also the liquid enclosed in the fiber is, however, given the opportunity of diffusing out through the fiber wall, which will further improve the washing result compared with those obtained in known washing up paratus in which the washing time is only for one or a few seconds.

Consequently, the method according to the invention means that the pulp is transported parallel to the screening surfaces, whilst the washing liquid slowly flows perpendicularly to the direction of transportation. In order to provide for a uniform fiow of the pulp suspension through the washing apparatus, Le. a flow without any substantial differences in speed of the various portions of the pulp in different parts of the washing zone, the pulp is transported intermittently or periodically in the apparatus. During a relatively short period of transportation the pulp is moved relatively quickly through a certain distance of the washing apparatus. During this period of transportation no extract is drawn off from the extract screen. The period of transportation is followed by a considerably longer washing period, during which the pulp lies still in relation to the screening surfaces. During the period of transportation the speed of the pulp in relation to the screening surfaces amounts to 0.020.5 m./sec., preferably 0.05 0.2 m./sec. The pulp can be transported through the apparatus by being moved by means of a disc or plunger a certain distance or by a certain quantity of pulp suspension being rapidly pumped into the inlet section of the washing apparatus. The invention 'will be described below in greater detail, reference being had to the accompanying FIGS. 1, 2 and 3 which in vertical cross sections show three embodiments of an apparatus for carrying out the method according to the invention.

According to FIG. 1 the apparatus is embodied as a short column, i.e. as an upright cylinder having a shell 1 and preferably circular-cylindrical coaxial screening surfaces 2 and 3, the screening surface 2 being intended for the supply of washing liquid and the screening surface 3 for taking out extract. The pulp is supplied through the inlet feed pipe 4 and is taken out through the outlet pipe 5 and over the baffie means 6. The pulp is transported through the column by means of the disc 7 which has the same contour as the cross section of the column at this place, to which disc is imparted a reciprocatory motion. Said motion can be effected by rapidly pumping liquid into the interspace between the disc and the end wall of the column through the feed pipe 8, or by moving the disc by means of external means (not shown), for example a piston. In the latter case the washing liquid is pumped through the pipe 8, so that the gap between the disc and the shell of the tower is kept free of fibers.

Washing liquid is supplied form the vessel 9 through the pipe 10 to the screen 2, and the extract is taken out from the screen 3 through the pipe 11 and the valve 12 and is supplied to the vessel 13, the liquid level of which can be controlled.

According to the invention the arrangement just described is utilized in the following manner. From a vessel containing unwashed pulp suspension (not shown in the drawing) a pulp suspension is fed continuously through the inlet feed pipe 4. The fibrous suspension may preferably have a fiber consistency of, for example, 10 percent. During the pumping-in operation the pressure in the upper portion of the column is so low that the pulp in the column clings to the walls, and the disc floats on the pulp and moves upwards in step with the amount of pulp pumped in. When the disc has reached its uppermost position, the valve 12 is shut and the disc 7, by a rapid motion pushes the pulp in the column downwards through a certain distance which is a fraction of the column length. During said downward motion, which takes place at a speed of about 0.1 m.sec., a valve may, if required, be shut within the supply line for the pulp before the inlet feed pipe 4.

When the disc 7 has reached its lowermost position, the pressure of the drive fluid is released at the feed pipe 8, the valve 12 opens and pulp is pumped in via the inlet feed pipe 4 while the disc 7 only very lightly rests on the pulp and is raised by the entering pulp in the way described above.

During this second phase, when the pulp clings to the walls of the column, the pulp is washed between the screening surfaces.

Owing to the fact that the liquid level in the vessel 9 is higher than that of the vessel 13, a pressure gradient is applied to the pulp layer between the intake screen 2 and the extract screen 3, said gradient moving the washing liquid from the screen 2 to the screen 3 at the same time displacing the liquid present in the pulp suspension. The liquid pressure gradient is kept as high as possible, but not so high that the pulp is, to any considerable extent, compressed against the extract screen 3.

When fresh, unwashed pulp has'again pressed the disc 7 to its uppermost position, downward transportation of the pulp in the column is again effected, by the disc being pressed downwards to its lowermost position, after which a new washing period is started etcetera.

The height of the column is chosen in such a manner that the withdrawn pulp suspension is well washed and contains only very small amounts of the liquid originally found in the fibrous suspension. The supplied quantity of liquid amounts to about 0.9-2 times the quantity of liquid originally contained in the unwashed pulp suspension supplied to the column.

As pressure liquid on the disc 7 it is possible to use extract received from the vessel 13.

The pulp consistency may be between 2 and 25 percent, preferably however between 3 and 15 percent.

The temperature may be chosen optionally to lie between +lC and above l00C In the latter case the column must operate as a pressure vessel and the vessels 9 and 13 must then be closed.

FIG. 2 shows another embodiment of an apparatus in which the screening surfaces are mounted as concentric screening bodies. 21 designates the cylindrical shell, and 22 a number of concentric screening bodies with intake screens 23 and extract screens 24. Pulp is supplied through the inlet feed pipe 25 and taken out through the feed screw 26. 27 designates a baffle means preventing the pulp from falling from the screening plates. Transportation in the axial direction is effected by the disc 28 which is moved downwards by liquid pumped in through the feed pipe 29. The washing liquid is fed to the intake screens via conduit 30, and the extract is taken out from the extract screens via the conduit 31.

The washing procedure is the same as the one described in connection with FIG. 1.

The method according to the invention may also be carried out in a column in which the movement of the pulp suspension is directed upwards or horizontally. in the former case the periodic transportation is effected by a disc in the manner described above or only by periodically pumping in a certain quantity of pulp suspension. The washed pulp is removed from the top of the column by means of a scraper or a feed screw.

FIG. 3 shows a third embodiment of an apparatus for washing fibrous suspensions according to the invention. The apparatus comprises a washing column, a so-called diffuser 41 consisting of a vertical cylindrical shell 42, a cylindrical screen 43 mounted within said shell and slightly spaced from the shell wall, and a perforated tube 44 mounted centrally within the column. The elements 42, 43 and 44 are coaxial. The screen 43 is intended to operate as an extract screen, and the screen 44 as an intake screen for washing liquid. Between the extract screen 43 and the shell 42 there are mounted, at a number of places, annular, horizontal walls 45 in order to prevent liquid from flowing in the axial direction of the column within the interspace between the extract screen and the shell. The column is therefore effectively divided into a number of successive screening sections. The interspace between the extract screen and the shell in each of these sections via adjustable valves 50 communicates with a conduit 53 for the extraction liquid, in which conduit an adjustable valve 51 is mounted.

Fibrous suspension is fed to the column through a conduit 46 and an adjustable valve 47 to the upper portion of the column, and fibrous pulp is taken out from the outlet 48 of the column via a conduit 49. Washing liquid is supplied to the interior perforated pipe 44 via a conduit 52.

As appears from FIG. 3, a plurality ofidentical diffusers can be connected in parallel with each other. in a plant for washing suspensions of cellulosic pulp the number of diffuser, connected in parallel, may amount to, for example, 2 -20, preferable 3 6.

The washing in the diffusers is carried out in the following manner. It is assumed that all of the diffusers are filled with fibrous suspension and that washing is going on by displacing the suspension liquid by means of washing liquid supplied to the interior perforated pipe 44 in a manner similar to that described above in connection with FIGS. 1 and 6. When the washing in one diffuser has been completed, the valve 47 is opened while the corresponding valves in the other diffusers, in which the washing has not yet been completed, are shut. At the same time as the valve 47 is opened, the valves 50 for out going extract are shut, by which pulp suspension is prevented from penetrating through the openings of the extract screen. In order to further safeguard against such penetration, it may also be possible, during the period when pulp is being pumped in, to supply a certain amount of liquid to the interspace between the extract screen and the shell. When pulp is being supplied through the valve 47, the washed pulp, present in the diffuser, is pressed downwards and leaves through the conduit 49 and is conducted to a storage tower for thick pulp or to a bleaching tower. The duration of the pumping-in operation preferably is chosen to be so long that the volume of the pulp quantity pumped in amounts to 1/20 l/2 of the volume of the diffuser. As soon as the desired quantity of unwashed pulp has been pumped in, the valve 47 is shut. Simultaneously the corresponding valve is opened in that one of the remaining diffusers, connected in parallel, in which the washing of the pulp suspension has just been completed. By controlling the diffusers, connected in parallel, cyclically in this manner, a substantially continuous flow of washed pulp can be obtained from the conduit 49.

The object of the partition walls 45 located between the extract screen and the shell according to FIG. 3, is as follows. For transporting the pulp through the diffuser a pressure gradient in axial direction is required. In the absence of said partition walls this pressure gradient may cause too great a part of the liquid in the pulp suspension to flow out through the extract screen at the upper portion thereof, to flow downwards in the interspace between the extract screen and the shell and, via the extract screen at the lower portion of the diffuser, to penetrate into the pulp suspension at that place. The main disadvantage thereof is that the dewatering of the pulp suspension at the intake portion of the diffuser becomes too strong, so that the consistency of the pulp suspension is no longer suitable for the type of diffusion washing forming the object of the invention. Moreover, the intake portion of the diffuser may become choked up. By the provision of the partition walls 45 these drawbacks are eliminated.

A battery comprising diffusers, connected in parallel, according to the invention, for example, of the type shown in FIG. 3, may according to the invention replace a conventional plant having filter washing apparatus between the various steps in a bleaching department, comprising a number of bleaching towers. Hereby the pulp from the top of a bleaching tower with upward flow of the pulp suspension may be caused to fall down to a thick pulp pump which cyclically pumps the pulp through the various diffusers of the battery in the manner described above. Subsequent to the washing the pulp is fed to the next bleaching tower which is an upward flow tower. By this arrangement it is possible to obtain a substantially continuous pulp flow both prior to and after the washing apparatus, and it is further possible to use one and the same thick pulp pump for feeding the pulp through the washing apparatus as well as for pumping the pulp into the subsequent bleaching tower.

The invention is not restricted to the embodiments shown in the drawing and the above described embodiments as these can be varied in different ways within the scope of the invention.

What we claim is:

1. In a fiber suspension washing apparatus comprising a column having a washing zone, means for introducing the fiber suspension to the column, means for introducing a wash liquid into said washing zone substantially perpendicularly to the direction of travel of the suspension therethrough, at least one inlet screen element and at least one extract screen element arranged coaxially in said column, and outlet means for discharging the washed suspension from the column, wherein the apparatus further includes means for moving the fibrous suspension intermittently through the washing zone of the column so as to provide dwell periods, during which the suspension is maintained stationary in said zone, and movement periods during which the suspension moves at a determined speed through said zone, means for applying a pressure gradient to the pulp layer between the inlet screen element and the extract screen element, and means for preventing excessive speed of the suspension through the column and ensur ing that the fiber suspension remains stationary between the screen elements during said dwell periods.

2. An apparatus according to claim 1 wherein the means for intermittently feeding the suspension through the washing zone to provide alternating dwell and movement periods comprises a movable disc disposed transversely of the main axis of the washing column so as to form a substantial seal with the walls of aid column.

3. An apparatus according to claim 1 wherein the pressure gradient applying means comprises a liquid-containing vessel having means for regulating the level of liquid therein and means connecting said vessel to the washing zone via the extract screen element.

4. An apparatus according to claim 1, wherein the means for preventing excessive speed of the suspension through the column and ensuring that the suspension remains stationary between the screening elements during said dwell periods, is in the form of at least one baffle means located in the close prox l imity of the fiber suspension outlet means.

5. An apparatus according to claim it, wherein said washing cylinder is cylindrical and further comprising a plurality of perforated screen elements for introducing wash liquid, a plurality of extract withdrawal perforated screen elements, said screen elements being cylindrical and arranged in spaced relation alternately with respect to each other and coaxially with respect to said cylindrical column.

6. An apparatus according to claim 1 in which said washing column extends vertically and the suspension is moved vertically downwardly therethrough, wherein means are provided for preventing downward movement of the suspension disposed between said means for introducing a wash liquid and said perforated screen element during said dwell periods.

Claims (5)

1. 2. An apparatus according to claim 1 wherein the means for intermittently feeding the suspension through the washing zone to provide alternating dwell and movement periods comprises a movable disc disposed transversely of the main axis of the washing column so as to form a substantial seal with the walls of aid column.
2. 3. An apparatus according to claim 1 wherein the pressure gradient applying means comprises a liquid-containing vessel having means for regulating the level of liquid therein and means connecting said vessel to the washing zone via the extract screen element.
3. 4. An apparatus according to claim 1, wherein the means for preventing excessive speed of the suspension through the column and ensuring that the suspension remains stationary between the screening elements during said dwell periods, is in the form of at least one baffle means located in the close proximity of the fiber suspension outlet means.
4. 5. An apparatus according to claim 1, wherein said washing cylinder is cylindrical and further comprising a plurality of perforated screen elements for introducing wash liquid, a plurality of extract withdrawal perforated screen elements, said screen elements being cylindrical and arranged in spaced relation alternately with respect to each other and coaxially with respect to said cylindrical column.
5. 6. An apparatus according to claim 1 in which said washing column extends vertically and the suspension is moved vertically downwardly therethrough, wherein means are provided for preventing downward movement of the suspension disposed between said means for introducing a wash liquid and said perforated screen element during said dwell periods.

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