WO1997010380A1

WO1997010380A1 - Method and apparatus for cleaning a fiber pulp washer from stickies

Info

Publication number: WO1997010380A1
Application number: PCT/FI1996/000482
Authority: WO
Inventors: Kaj Henricson; Pekka Kaarakainen; Seppo Kokkonen; Anssi Luostarinen; Harri Qvintus; Erkki Savolainen; Pekka Tervola
Original assignee: Ahlstrom Machinery Oy
Priority date: 1995-09-11
Filing date: 1996-09-11
Publication date: 1997-03-20
Also published as: FI960438A0; FI960438A; EP0853700A1

Abstract

An apparatus and methods for cleaning a fiber pulp washer from stickies. The apparatus comprises a stationary casing and a rotatable drum, disposed inside the casing and having a liquid-pervious surface (7) having holes, slots or corresponding openings. Filtrate compartments (17), having side and bottom surfaces, are disposed inside the surface. The liquid-pervious surface of the drum comprises a number of replaceable elements (21). Cleaning wash methods, whereby the washer is disconnected from normal displacement wash process, are effected by feeding a cleaning wash liquid in the same direction as the wash liquid of the displacement wash process in a closed circulation. Other cleaning wash methods, which are continuous, are effected by supplying cleaning wash liquid onto the filter surface of the drum through nozzles, which nozzles are either moved continuously in the axial direction of the drum or are disposed in the axial direction along the whole length of the drum.

Description

METHOD AND APPARATUS FOR CLEANING A FIBER PULP WASHER FROM STICKIES

The present invention relates to a method and apparatuε for cleaning a fiber pulp washer from sticky substances, i.e. stickies. In the manucture of paper pulp, chemicals are used which release organic compounds from wood material. The purpose of the washing process is to remove as much of this liquor solution produced as possible with a minimal amount of wash liquid so that the pulp is clean enough for further treatment, washing the pulp is one of the most important partial processes in a sulphate pulp mill. The wash aims at: separating liquor from pulp, recovering the liquor in order to regenerate cooking chemicals and to combust combustible substances dissolved in the liquor; and reducing the efflued load from the mill. Further, washing the pulp is necessary in view of the runnability of subsequent processes. Dirty pulp causes problems, for example in screening it foams. Also chemical costs (e.g. anti-stickies agents) in bleaching and paper manufacture will increase. The most important physical basic operations in washing pulp are dilution, thickening, displacement, diffusion, flotation and mechanical labor. Displacement wash is clearly the most efficient washing method. In connection with the method and apparatus of the present invention, cleaning a pulp washer, i.e. washing the filter surfaces of a pulp washer, is for clarity called a cleaning wash.

In connection with the invention, the term stickies mean all different substances which tend to plugg the apertures in the filter surfaces of a pulp washer. Thus, for example in addition to resins also silicates, oxalates, and other possible substances forming deposites which cause plugging of the apertures of the filter surface are covered by the term stickies. The invention is particularly well applicable in connection with a so-called DD washer (DRUM DISPLACER^®) of A. Ahlstrom Corporation, and it may be used also in other pulp washers such as washing presses where applicable. A DD washer is a pulp washing apparatus employing displacement; in the apparatuε, wash liquid is forces through a pulp cake of a consistency of approx. 10 %. During the displacement wash stage the pulp wash liquid continuously flows through the pulp cake collected onto the surface of a rotating drum by means of a pressure diffecence and displaces the dirty liquid (liquor) in the cake. Thus, the liquid (liquor) in the cake iε replaced by a cleaner liquid. The pulp washing process may be carried out in one and the same drum from one to four times depending on the number of stages of the washer. A DD washer operates on the countercurrent principle, i.e. filtrate from the last washing stage is used as wash liquid in the second last washing stage, etc. Usually, in addition to the wash in a digester, 3 - 4 pulp washing stages are needed for washing brown stock.

Figure l illustrates schematically the operation principle of a two-stage DD washer representing prior art technology. In a DD washer, fiber pulp is washed by pulp wash liquid flowing through the pulp. The flow-through takes place via perforated plate sectors in the surface of a drum l (partial enlargement in Fig. l) . Pulp is washed in 1 - 4 stages, depending on the structure of the washer, according to the so-called countercurrent principle. Filtrate from a cleaner pulp washing stage (i.e. the subsequent washing stage) is used as the pulp wash liquid in each stage. Filtrate flowing into filtrate compartments is circulated countercurrent relative to the revolving direction of the drum. The wash liquid in the las washing stage is as clean process water 13 as possible. In Figure 1, reference number 10 refers to a filter unit which cleans the filtrate used in the last wash liquid feed point for use in the second last wash liquid feed point. The operation principle of the washer has been described in detail for example in Finnish patent no. 74752 and corresponding U.S. patents no. 4,919,158 and 5,116,423.

A DD washer mainly comprises a rotating drum 1, a stationary casing 20 surrounding the drum, and a distribution valve 25 (illustrated in Fig. 6) disposed either in one end or both ends of the drum. The rim l of the drum has been divided by intermediate walls in feed chambers, each of which comprises an inlet for the wash liquid used for washing pulp. Wash liquid is guided from wash liquid feed chambers to the pulp in pulp compartments 24 to displace the liquid therein. The bottom of the feed chambers is made of a liquid-pervious filtering surface, most commonly perforated plate. The surface of the rim l of the rotating drum of a DD washer illustrated in Fig. 1 iε provided with a filter surface 7 (e.g. perforated plate) , which has been divided into compartments with dividing ribs 2 which are 50 - 70 trim high and disposed at 200 - 300 mm spacing. These dividing ribs form with the perforated filter surface the so-called pulp compartments 24, and between the filter surface and the rim of the drum so-called filtrate compartment/compartments 17 (Fig. 2) .

At the end of the cylinder drum substantially at the diameter of the drum rim, there is a valve arrangement

(distribution valve 25) through which filtrate is removed from the filtrate compartments and is transported further for example in pipe lines 5 or 12.

Outside the drum, there are sealing elements 3 mounted in the casing. In the feed stage A of a DD drum, pulp is fed as illustrated by arrow 11 in Fig. 1 at a pressure of approx. 10 - 50 kPa. The consistency is about 4 - 10 % depending on the process. Pressurized pulp is compressed onto the surface of the perforated plate 7 of the drum and fills the compartments defined by the dividing ribs 2 and simultaneously filtrate flows through this perforated plate. When the drum revolves and the compartment passes the first sealing element 3A, excess pulp which is higher than the dividing rib is swept away and the thickness of the pulp cake becomes constant. Filtrate obtained from the first washing stage B of a DD washer is usually extracted as one fraction and it is taken via line 12 to pulp dilution and/or chemical recovery. When the pulp enters the first washing stage, the thickness and porosity of the pulp cake are homogenous and the consistency iε approx. 10 - 12 %. In the subsequent washing stages, filtrate from a cleaner stage is used to displace the dirtier wash liquid in the pulp cake in the previous stage and the filtrate is taken via pipe lines 5 to be used as the displacement liquid in the previous, dirtier stage; thus for example in Fig. l from washing stage C to washing stage B.

The pressure of the wash liquid is usually of the order of approx. 0 - 100 kPa (l bar) because strength calculations are made for a liquid pressure of 100 kPa but in future constructions, for example a pressure of about 120 kPa is probably quite possible. The pressure of the washing filtrate may be slighly increased between the washing stages by circulation filtrate pumps 8. The pressure of the wash liquid and the filtrate pumps provide the pressure for the entire wash and filtrate circulation which preventε air from being mixed into the pulp.

The pulp passes from the last waεhing zone to a discharge zone D. When the pulp cake is by the last sealing element the connection to a suction pump of the filtrate duct is opened and filtrate remaining in the flow duct under the perforated plate is .removed by the pump. When the pulp cake has passed the suction point a pressure air pulse (pressure air duct 9) is directed in the compartment under the perforated plate and the pulp cake is detached from the perforated plate. The pulp drops onto discharge a screw 6 to be taken to a following process. Reference number 10 refers to a filter unit employed in cleaning the filtrate used in the last wash liquid feed point for use in the second last wash liquid feed point.

When a normal process is running the drum of a DD washer is cleaning washed, after detaching the pulp from the surface, with a wash liquid having a pressure of 800 - 1000 kPa (8 - 10 bar) ; in future new strength calculations and more appropriate materials perhaps allow a maximum pressure of about 1200 kPa (12 bar) . Further, the washer is provided with a so-called high-pressure wash duct capable of reaching a pressure of about 8 - 10 MPa (80 - 100 bar) . What was estimated above to take place in connection with the cleaning wash in a conventional pulp washing process applies also for development of the pressure limits in the high-pressure cleaning wash. In future new strength calculations and more appropriate material choises may allow pressures of up to about 14 MPa (approx. 140 bar) for high-pressure cleaning wash. The cleaning wash of the perforated plate may be intensified from time to time for example by means of jets from a high-pressure washing duct, for example at wash cycles of 5 minutes. In this case the normal displacement wash process has been disconnected and the washer drum is rotating. Also this cleaning wash has an effect primarily on the filter surface.

Figure 2 illustrates a projection of the end of a prior art washing drum, illustrating depicting schematically securing of the perforated plate of a DD washer. The perforated plate 7 is supported towards the center of the drum by a supporting plate 14. The supporting plate again comprises a metal plate 16 forming the filtrate duct, and vertical supports 15 welded thereto. The bottom of the supporting plate 14 rises in the direction away from the distribution valve. The purpose of this is to make the flow of wash liquid through the pulp cake more even in the longitudinal direction of the drum. At sides the perforated plate is supported by axial dividing ribs 2 which divide the rim 1 of the drum into sectors. The perforated plate is prevented from moving in the direction away from the center of the drum by support blocks 18, 19 welded in the end of the drum and in the dividing ribs 2. During erection, the supporting plate 14 and its vertical supports 15 are places as backing for the perforated plate 7. After this the perforated plate is placed against the supporting plate and the supporting blocks 18, 19 supporting the perforated plate are welded on. Even originally, the perforated plates are not designed to be replaced. Therefore, when in practise it has been necessary to replace the perforated plate because of damages, the replacing has been a very difficult operation. When a perforated plate is replaced the supporting blocks 18, 19 supporting the perforated plate must be detached, in other words the welding seams must be opened. After this, the damaged perforated plate is transported away and a new plate is installed in its place. Working conditions during the replacement of the perforated plate are, however, very inconvenient. The space is wet and the temperature prevailing depends on when the washer has been disconnected from operation and how well it has cooled down (the running temperature is about 100°C) . The only spot where the perforated plate may be reached is the maintenance opening of the screw transporter 6 under the DD washer. The operations necessary for the replacement (grinding, welding) must be carried out on top of the screw. The work is further made more cumbersome by the fact that the perforated plate must be treated working upside down on top of the screw; thus the working position is very difficult (armε get tired easily and welding and grinding sparks fall on the erector) . Further, only the space between the screw 6 and the surface of the drum is available for movements and even this is limited by for example the pipelines of the washing apparatus cleaning the surface of the drum. An alternative way of replacing the perforated plate iε to remove the block εurrounding the drum (the upper block) . However, this method is very time-consuming and requires high crane capacity.

An object of the present invention has been to provide an alternative perforated plate construction which is better in view of the maintenance of the perforted plate.

Replacing a perforated plate may be necessary not only for purely maintenance reasons but also for mechanical reasons, such as too intensive bulging of the perforated plate in connection with the pressure air pulεe applied when detaching the pulp cake. Also fatique and reasons caused by the cyclic fluctuations in the temperature differences as well as various combination of these sometimes cause the need to replace the perforated plate by a new one.

Another object of the present invention has been to find a method of washing fiber pulps which are difficult to wash. Until now a DD washer has not been intended for washing pulps containing silicates or plenty of other sticky substances. It is common belief that probably the stickies would quickly plugg the apertures in the perforated plate. Paper industry has poor experiences for example of cleaning plugged perforations of suction rolls. The aperture size used in the perforated plate of a DD washer is smaller and there are more apertures in the plate. Cleaning of a severely plugged perforated plate would be difficult and would take much time. A particularly difficult problem in bleaching is caused by oxalate deposits. It has been estimated that about 10 - 20 kg of oxalateε per ton of pulp iε produced in a bleaching proceεε. If the effluent volume of the bleaching proceεε iε εmall, leεε than 5 m³/ton pulp, the oxalates begin to be accumulated in the bleaching system and cause deposits. This formation of deposits may be very rapid.

In future, fiber pulp may originate in addition to the conventional softwood trees (long-fibred pulp) and birch

(short-fibred pulp) to an increasing extent also from for example bamboo, eucalyptus and reed. Fiber pulps manufactured from the raw materialε available contain varying amounts of silicateε and different stickies, such aε resin etc.

Thuε, both existing maintenance reasonε and future procesε prospects have given rise to the present invention.

The invention aims at solving the problems present in the existing apparatuε and washing methods and providing improved methods and apparatus applicable in many variouε washer types and capable of reaching almost the optimal washing result typical of each washer and procesε type and fiber pulp raw material.

The charactiεtic featureε of the apparatus of the present invention are discloεed in the appended patent claimε. The method and apparatus of the present invention are described more closely, by way of example, with reference to the accompanying drawing figures, of which

Figure l illustrateε schematically the operation principle of a prior art 2-stage washer;

Figure 2 illustrates a projection of an end of a prior art washer drum, disclosing schematically securing of the perforated plate of a DD washer,- Figure 3 illustrates schematically a filter element according to the invention;

Figure 4a illustrates a preferred embodiment of an element according to the invention, based on the use of a wire;

Figure 4b illustrates a section along line A-A of the element of Figure 4a;

Figure 5 illustrates a tool for use in connection with the replacement of an element;

Figure 6 illustrates an opening in the end of a washer for use in the replacement of an element according to the invention;

Figure 7 illustrates an embodiment in which the thermal expansion of an element according to the invention has been taken into account;

Figure 8 illustrates a construction for carrying out the cleaning wash method of the invention; and

Figure 9 illustrates another construction for carrying out the cleaning wash method of the invention.

In an improved DD washer according to the invention, the filtering surface is formed by an element which may be easily and quickly replaced if necessary. In addition to this, special attention has been paid also to the cleaning wash stage, i.e. the stage between the discharge of the pulp web and the feeding of new pulp, by improving also the construction of the machine elements required in this important process stage, the constructions being described closer in the description of a cleaning wash method disclosed later. The intensified cleaning wash method of the present invention provides a solution also to cleaning problems of the entire washing apparatus system, including filtrates pipe lines, pumps, wash liquid feed chambers, pulp feed zones etc. Further, special attention has been paid to keeping the washer drum clean while the process is running, i.e. during operation, particularly when processing difficult fiber pulps rich in stickies. The intensified on-line cleaning wash method of the present invention provides a solution also to these problems. For example, the operation of a DD waεher may be εignificantly improved by employing the method and appratus of the invention. Reliable operation and ease of maintenance of the washer and properties of the procesεed pulp may be improved εignificantly by applying the present invention. Using the present invention, processing of fiber pulps is possible which previously have not been treated in a DD washer, or their treatment has been extremely difficult for example due high stickieε contentε. Remarkable improvements are obtainable also in processing of conventional fiber pulps by applying the present invention. In this context, procesεing meanε both waεhing and bleaching proceεεes.

When liquid circulations in a bleach plant are closed the concentrations of organic and inorganic compounds present in the pulp and filtrates begin to increase and result in corrosion in filter surface 7, dividing wall 2 and in the bottom of filtrate compartments 17. Another problem with material and design is that compounds begin to thicken and stick onto the surfaces mentioned that possibly cause corrosion or reduce the washing capasity of the washer to wash pulp efficiently, by plugging the openings in the filter surface 7. If the surface has been plugged, installing a new filter surface and dividing walls takes time and is a very costly operation. This drawback may be eliminated and the problem solved by manufacturing elements according to our invention which have been illustrated schmatically by reference number 21 in Figure 3.

Under the filter εurface 7 of a cylindrical waεhing drum there are longitudinal filtrate compartmentε 17 defined by axial intermediate wallε 15. Said dividing ribs 2 extend outside the surface l of the drum so that they divide the fiber pulp to be washed and therefore supplied against the surface of the drum in substantially rectangular cakes, or planks. This element 21 between the dividing wallε 2 comprises one or sevaral filtrate compartmentε 17 formed of a liquid-perviouε filter surface 7 which may have perforations, slots or corresponding apertures, a wire surface etc, serves as a cover, and forms a subεtantially cylindrical shell; and of one intermediate wall or several intermediate walls 15 and of a (metal) plate 16 forming a filtrate duct. The plate 16 forming the filtrated duct may be made of metal or other suitable material such as a composite of metal, cheramic or plastic material, or the like. If there are material or other problems discussed above the element may be pulled out via the end of the washer and a new replacing element pushed in place. This new method allows a remarkably quicker and cheaper maintenance of the washer.

This new element serving as a filter surface may be effected for example as based on the use of a wire. Also various slot or perforation elements may be utilized in the filter surface according to the invention.

An element must be replaced if it has been damaged εo that too much fibers pass through the filter surface or if the surface in very demanding conditions with time is clogged permanently/to a harmful extent deεpite the uεe of efficient washing methods. A damage is detected from a too high fiber content of the thickening filtrate and clogging from a high pumping pressure of the wash liquid.

Securing the element 21 to the surrounding structure may take place for example by meanε of a εpring 22 welded in the end of the element aε illuεtrated in Figure 4b. The contact εurface of the element and the drum is large and fibers are collected with time into the slots between. Further the element may be damaged by a hard particle which results in minor deformations. Due to this, the element may be tightly caught and the use of great force may be necessary in replacing the element. Because of this, particular attention has been paid to the securing/replacing means of the element. A construction as illustrated in Figures 4a and 4b has been found to be good in connection with the element of the invention. An element iε attached to the drum by puεhing via the end of the drum. There are springs 22 welded in the end of the element which "click" into a groove 23 cut in the end of the drum as illustrated in Figure 5 and lock the element 21 in its place. When replacing an element the element is pulled from the locking springs welded in the element for example by means of a tool 38 developed for this purpose and illustrated in Figure 5.

An element is replaced via a securing hole of a closing piece 39 of the distribution valve 25 illustrated in Figure 6.

An element 21 is replaced in the following stages: l. Pulp feed must be collected to a blow tank and the filtering surface muεt be cleaned with a washing means. 2. The window (not illustrated in the Figures) in the side of the discharge block is opened and the damaged element 21 is detected visually.

3. The element 21 is marked so that it may be clearly seen from the end of the drum.

4. The closing piece 39 in the distribution valve 25 iε detached and the drum iε rotated until the marked element 21 is detected via the opening of the closing piece.

5. The element 21 iε pulled out by uεing a tool 38 deεigned for the replacement.

6. Fibers which have adhered to the surface of the drum are cleaned off for example by means of a water jet of a scraper.

7. A new assembled element 21 is installed. 8. The closing piece 39 of the distribution valve and the window of the discharge block are closed. 9. Pulp feed may be commenced.

The own maintenance group of the pulp mill may replace the elements and the time the work requires is so short that the pulp digestion process need not be interrupted. Working conditions in the replacement operation are essentially better than in replacing the prior art filter elements.

Figure 4 illustrates the structure of a filter element 21 in which the cover of the filter compartment is manufactured of a bendable surface. A bendable surface like this may be effected for example by means of a wire. in this case the wire element comprises a bag 26 made of the wire material one end of which has been cloεed for example by εewing. A εupporting plate 16 εerving aε the wire element body, is disposed inside the wire bag 26 and it guides the wash liquid to a distribution valve 25 at the end of the drum, inside the wire bag 26 on top of the supporting plate there is a sparse perforated plate 27 preventing the wire from hanging due to the pressure of the pulp cake. The wire is secured in its place by means of a rib 28 placed at the end of a filtrate opening. The wire bag 26 surrounding the structure formed by the supporting plate 16 and the perforated plate 27 is made of εuch a εhrink material that it will withstand both the temperature of the waεher and the influence of the chemicalε uεed in the waεher. The wire is presεed with screws 29 between the perforated plate 27 and the rib 28. The surrounding structure prevents the screws 29 from being detached and damaging for example filtrate circulation pumps 8. The wire material may be for example PVDF, polyvinylidene fluoride, a plastics thread material which tolerates the operation temperature of 100°C of the washer and the chemicals used in the waεh. PVDF is a fabric sleeve material and it shrinks aboun 10 % when heated to 100°C.

The supporting plate of the element has been provided with vertical supports 15, which have been for example welded with laser, and support the perforated plate and the wire against the pressure of the pulp cake. The bottom of the supporting plate is treated by bending or pressing to form a bulge 30 which is used instead of the rise of the bottom in the prior art structure. The heigth of the bulge decreases evenly towards the filtrate opening. Thuε, by the filtrate opening only the vertical supports 15 and the springε 22 keeping the element in place provide cross- sectional area preventing the flow. The flow area of the filtrate increaseε and the flow is linear.

The filter surface or the cover of the filtrate compartment, which in the example presented here is a wire bag made of a wire, may be replace, if desired, by a stiff filtrate compartment cover such as slot plate or a perforated plate while the rest of the element structure remains mainly the same as above.

Particularly in view of the properties of future fiber pulps mentioned above (which contain various stickies such as silicates, oxalates, reεin etc, which tend to clogg the openings of the filter surface) it is preferable if for example a filter surface element effected by means of a wire may rapidly be replaced by an element effected by means of a slot plate or a perforated plate.

The structure of a εlot plate and a perforated plate element greatly reεembles the structure of a wire element.

The replaceable element comprises in this case a stiff surface having holes, εlotε or the like apertureε serves as the filtrate compartment cover. A stiff surface means here a surface made of a material which does not require a separate sparεe perforated plate disposed on top of the supporting plate to prevent the wire from hanging under the presεure of the pulp cake which is required when a bag made of wire is used as the cover of the filtrate compart¬ ment. Thus, normal prior art surface materials and plates used in conventional fiber pulp washer drums may be used as the stiff surface of the filtrate compartment of the element of the invention.

In an embodiment of the invention utilizing slot or hole plate elements, a εupporting plate guiding wash liquid to a distribution valve located at the end of the drum serves as the body of the element. Vertical supports which carry the cover of the filtrate compartment against the pressure of the pulp cake have been for example welded to the supporting plate of the element. Prior art apparatus do not pay adequate attention to transition caused by thermal expansion. Thiε haε been one of the factorε initiating damages. The structure according to the invention allowε also for thermal expansion and it may freely take place because the securing of the element permits the element to move the distance that is required by the thermal expansion. This allowance of the element haε been illuεtrated in Figure 7 with reference nuumber 31.

The invention relates to an improved method during the application of which the pulp washing apparatus is disconnected from the process. This method pays special attention to the cleaning wash εtage, i.e. the stage between the discharge of the pulp web and the feed of new pulp, by improving alεo the conεtruction of the machine elementε required in thiε important proceεε εtage.

According to the invention, a DD waεher may be provided with new washing apparatus most of the components of which, however, already exist. By means of thiε new washing arrangement all the components of a fiber pulp washer may be cleaned which come in contact with the fiber pulp to be washed during normal displacement wash of fiber pulp. Thus for example all pipe lines, pumps, perforated plates, filter surfaceε, pulp feed and tranεport ductε may be cleaned from material adhered thereto.

The method of the invention is effected by a cleaning waεh apparatuε in which the diεcharge zone haε been carefully iεolated from other zones by means of sealing elements, i.e. in practice from the pulp feed zone and the last washing zone. Further, the cleaning wash apparatus comprises a cleaning wash liquid pipe line and the required vessels and pumps. If necessary, cleaning wash liquid may be brought to the system straight from outside via a separate feed and discharge pipe line. The cleaning wash liquid pipe line and the required vesselε and pumps and the wash liquid pipe line of the washer itself with the required pumping meanε, used in the displacement wash of fiber pulp and known per se, constitute the cleaning pipe line. The cleaning wash method according to the invention may be effected for example by means of the following cleaning wash system illustrated in Figure 8.

The discharge zone has been carefully isolated from other zones by means of sealing elements 3.

A separate tank 32 with required pumps and cleaning liquid pipe lines 33, 5, 34, 35, 36 have been constructed in connection with the fiber pulp washer and connected via appropriate valves to the wash zones of a DD washer, from which wash liquid is guided during displacement wash to a previous wash zone. From this spot on, the same pipe lines and pumps are used as in a normal diεplacement waεh of fiber pulp. The pipe line of the first wash zone of a DD washer has been extended by means of an appropriate valve so that the wash pipe line 34 reaches also the fiber pulp feed box 37. Between the wash line and the fiber pulp feed box there is alεo an appropriate valve. Figure 8 doeε not illustrate these valves which are necessary to make the cleaning wash liquid flow in the pipe line in the way desired. Their location is obviouε to a perεon of ordinary skill in the art. The wash pipe line discharge duct 36 from the feed box 37 has been connected to the discharge duct 35 of the wash liquid of the starting end washing zones of a DD washer. This entity has then been connected to the tank 32 in which the contaminated cleaning wash liquid is cleaned and returned for recirculation. In practice the cleaning is effected by disconnecting the washer from the normal procesε and by washing the surface of the drum according to an ordinary high-pressure washing program. Subsequently, cleaning liquid is circulated via tank 32 and pipe lines 33, 5, 34, 35, 36 through the washer which has been stopped. Thus, the cleaning wash liquid washeε al oεt all the memberε, i.e. pipe lineε, pumps, filter surfaces, perforated plates, pulp feed zones, etc. which participate in the displacement wash process in the washer. The cleaned liquid is either returned to the same vessel from which it was taken to the circulation, or guided somewhere else for proceεεing. When a firεt half of the washer has been washed the washing iε terminated and the waεher drum iε rotated half a cycle. After this, the wash of the drum is started again. When also the second half of the drum has been washed the washer may be connected for normal use for washing pulp. Cleaning wash liquid is fed, in addition to one pipe line or several pipe lines of the displacement wash procesε, alεo via the pulp feed line of the displacement wash process, if necesεary, in order to ensure that the entire pulp treatment apparatus including the pulp feed line and the by-pass line of the pulp feed are cleaned. When cleaning liquid is circulated through the same apparatus which is uεed in normal displacement wash of fiber pulp, even the parts of the displacement wash system which until now have been difficult to clean, are cleaned. Areas of this kind which are excluded from prior art cleaning waεh methods are for example all pipe lines of the washer, pumps, ducts, casings, and the pulp feed pipe line.

When inspected as a broader entity, the cleaning wash method of the present invention for cleaning a fiber pulp washer from stickies according to which the washer, e.g. a DD washer, is disconnected from the normal displacement wash process, comprises at least the following stages. Cleaning wash liquid is circulated from the tank 32 or directly from a separate wash liquid pipe line via one pipe line or several pipe lines of a washing stage/stages of the displacement wash proceεs onto the surface of the washer drum on the same principle as in the displacement wash process, i.e. in the same direction as in the displacement wash C -> B. In this way, in addition to the washer drum and the adjoining members, also the entire wash liquid pipe line 5, pulp feed box 37 and pumps 8 used in the displacement wash are cleaned. The wash liquid used in the wash of the washer is returned to the circulation tank 32 for cleaning or is pumped/guided elsewhere for procesεing. If neceεsary the washer drum is rotated after the first cleaning wash stage gradually onwards and the cleaning wash is continued by supplying cleaning wash liquid through the washer drum shell untill the whole drum shell is clean. If all the pipe lines of the displacement wash are utilized which is natural the cleaning wash of the entire washer apparatus may be carried out in two stages as described above. After the cleaning the washer, e.g. a DD washer is reconnected to the normal displacement wash process.

As already stated above, cleaning wash liquid is supplied via the pulp feed pipe line of the displacement wash process in order to clean the entire washer apparatus.

When the discharge zone has been separated from the other zones by means of sealing elements the entire cleaning wash ocess of the invention may be effected as a closed circulεXon procesε. Thuε there iε no danger of the waεh liquid from leaking to the fiber pulp dicharge zone D.

The influence of the cleaning waεh liquid is based primarily on its ability to dissolve stickies. The cleaning wash may be carried out at first with an acid waεh liquid and after that with an alkaline wash liquid or only with an acid wash liquid.

Figure 9 illustrates a simpler way than the basic idea of "entire apparatuε waεh" deεcribed above for carrying out the cleaning wash. Thiε embodiment is a reduced verεion of the idea of the entire apparatus wash and in this embodiment cleaning wash liquid is supplied only to those washing zones from which filtrate produced during normal displacement wash is directed straight to further- treatment, for example combustion (pipe line 12, Fig. 1). In this case, the drum of the washer must be rotated onwards in stages several times until the whole filter surface of the drum haε been cleaned. However, the structure of the cleaning wash apparatus required is remarkably simpler than the one required for the "entire apparatus wash" . Figure 9 does not illustrate all the valves required for ensuring the flow of cleaning wash liquid to the desired places in the washer; the location of the valves is obvious to a perεon of ordinary εkill in the art. The cleaning wash is preferably effected in connection with a normal high-preεεure waεh of a washer drum. At that time the surface of the drum is washed by a high-pressure wash apparatus known per se. When the first high-pressure washed drum εurface portion rotates to a reduced cleaning waεh zone the εame drum εurface portion is washed again by the reduced wash method.

This reduced wash apparatus may be used for example when stickieε tend to be collected to a harmful extent onto the filter surface of a washer drum faεted than in other memberε of the displacement wash apparatuε.

Thus the object of this cleaning wash is primarily to clean the drum surface. Naturally, also the perforated plate at the cleaning wash zone and the portion of the washer pipe line which is used both in the normal cleaning wash and in this method for circulating the cleaning wash liquid, are cleaned at the same time.

An object of the invention is also an improved method during application of which the pulp washing apparatuε operateε while a normal pulp waεhing proceεε is running.

This continuous cleaning wash of the drum of a pulp washer from stickies takes place in the zone between the removal of pulp web and the supply of new pulp.

This method of cleaning a fiber pulp washer from stickies is practiced by an apparatuε compriεing a εtationary casing; a rotatable drum, dispoεed inside the casing and having a liquid-pervious surface, i.e. filter surface composed of one entity or elements, and having holes, slots or corresponding openings, filtrate compartments disposed inεide the surface and having side and bottom surfaces and intermediate walls extending outεide the filter surface; a perforated substantially cylindrical surface disposed surrounding the drum, chambers with side and cover surfaces being disposed outside the perforated surface. This cleaning wash apparatus further comprises one high-pressure nozzle or several adjacent high-pressure nozzles movable in the axial direction of the waεher drum and diεposed separately following the normal cleaning wash zone of the washer, i.e. after the space where the wash nozzles are located which are used in conncection with a normal cleaning wash in a fiber pulp diεplacement wash stage, viz. in the wash following the detachment of pulp cake. High-presεure feed nozzles may alternatively be εtationary diεposed along the whole length of the washer drum in its axial direction. This stickies removal apparatus operating continuously has been isolated by appropriate εealing elementε from the feed zone of the pulp to be washed and from the discharge zone of washed pulp/normal wash zone. This cleaning wash apparatus designed for continuous operation of removal of stickies also comprises a cleaning wash liquid pipe line with required tanks and pumps. If required, cleaning wash liquid may be introduced directly from elsewhere via a εeparate feed and return pipe line. The waεh liquid used in the cleaning wash of the washer is reciculated to a circulation tank for cleaning or it is transported for treatment somewhere else.

According to this method, high-pressure cleaning waεh liquid is supplied continuously onto the filter surface of the drum on the area defined by the operation range of the high-presεure nozzleε.

In the cleaning waεh method employing movable high- pressure nozzles, high-presεure cleaning waεh liquid iε εupplied onto the filter surface of the drum on the area defined by the operation range of the high-pressure nozzles while the high-pressure nozzles are moved continuously in the axial direction of the drum, whereby a spiral area of the drum shell is continuously cleaned the width of which is determined by the coverage of the high-presεure nozzleε, or in stages whereby a zone of the drum shell is cleaned the width of which is defined by the coverage of the high-preεεure nozzles. The wash liquid used in the cleaning wash of the washer iε returned to the circulation tank for cleaning and reuse or it iε pumped/guided to be processed somewhere else or the liquid iε guided via a εeparate discharge pipe line to be processed somewhere elεe.

When employing this cleaning wash method the principle of closed circulation may be applied advantageously. The method is well applicable in removing oxalate depoεites from a bleaching plant where they exist abundantly and continuously.

When the method of stationary nozzles is used, high- preεεure cleaning wash liquid is supplied onto the filter surface of the drum to an area determined by the coverage of the high-pressure nozzles. The wash liquid used in the cleaning wash of the washer is returned to the circulation tank for cleaning and for reuse or it is pumped/guided to be processed somewhere else. When a separate cleaning wash pipe line is employed the liquid iε guided via a separate discharge pipe line to be procesεed somewhere else. If the operation conditions allow for example the following arrangement is advantagous for minimizing the amount of wash liquid used. The operation of the high- pressure nozzles is connected to a control logic which activates a part of the nozzles at a time. In this way the waεh water amount needed may be reduced remarkably. Thiε arrangement, however, presupposes that the properties of the fiber pulp to be treated are such that continuous operation of the high-pressure nozzles is no necessary to keep the surface of the drum clean.

when employing this cleaning waεh method the principle of closed circulation may be applied advantageously. The method is well applicable in continuous removal of oxalate depositeε from a bleaching plant.

When the new type of perforated plate or perforated plate element and the new improved cleaning waεh method of carrying out the wash while the pulp washing proceεs is running or the new improved cleaning wash method of carrying out the wash while the pulp washing process has been interrupted, are combined an improved DD washer of a new type is obtained. A DD washer of this new type may be used in washing also pulp materials, for example fiber pulps rich in silicates or other stickieε, which DD washers or other corresponding washers having small perforation diameters have previously not been able to treat. The solutions provided by the invention, both the apparatuε and the cleaning waεh method, may be uεed in connection with treatment methods of fiber pulp in which DD washers have not been applicable before. Such treatments are for example bleaching and treatment of fiber pulp in a closed circulation for example in connection with washing and/or bleaching stageε in a fiber line.

The inventive idea of the present invention may be employed in addition to DD washers also in washing presses where applicable.

As may be understood from the above description, washing processes and apparatus of wood procesεing induεtry have been developed by the present invention to permit washing and/or bleaching of fiber pulpε which differ from previouε pulps and particularly which are "difficult" and will become more common in future, in DD washerε or corresponding apparatuε without the riεk of εtickieε affecting to a too harmfull extent the proceεεing of the fiber pulpε. When the application range of DD waεhers becomes wider the fiber pulp treatment line becomes much more versatile in view of the fiber pulp to be washed, and closing the circulation makeε the εystem more economical and environmentally friendly. It should, however, be noted that the embodiments deεcribed above are only a few preferred alternative embodiment exampleε of the invention which do not intend to limit the εcope of protection of the invention which iε defined by the appended patent claims.

Claims

We claim:

1. Apparatus for cleaning a fiber pulp waεher from εtickies comprising a stationary casing; a rotatable drum, disposed inside the casing and having a liquid-pervious εurface (7) having holeε, εlots or corresponding openingε, filtrate compartmentε (17) , disposed inside the surface and having side and bottom surfaces, and intermediate walls (2) extending outside the surface; a perforated substantially cylindrical surface, disposed surrounding the drum, outside the surface of which there are chamberε with εide and cover εurfaces, characterized in that the liquid pervious surface of the drum comprises a number of replaceable elements (21) .

2. Apparatus as claimed in claim 1, characterized in that the replaceable element (21) comprises one filtrated compartment or several filtrate compartment (17) formed by the surface (7) having holes, slotε or correεponding openingε and serving as the cover of the compartment/compartments, by intermediate walls (15) and a plate (14) serving as the bottom of the filtrate compartment.

3. Apparatus as claimed in claim l, characterized in that the surface (7) having holes, slots or corresponding openings and serving as the cover of the filtrate compartment (17) of the replaceable element (21) is bendable and supported by appropriate means, for example with a sparse perforated plate (27) or the like means εo aε to prevent the bendable εurface from hanging under the pressure of the pulp cake.

4. Apparatus as claimed in claim 3, characterized in that the surface εerving as the cover of the filtrate compartment of the element is made of a wire.

5. Apparatus as claimed in claim 4, characterized in that the replaceable element comprises

- a bag (26) made of wire material one end of which haε been cloεed for example by εewing or some other suitable method; supporting plate (14) dispoεed inεide the wire bag and serving as the wire element body and guiding the wash liquid to a distribution valve (25) at the end of the drum via which filtrate is discharged from the filtrate compartment (17) and is transported further;

- a sparεe perforated plate (27) , diεposed inside the wire bag on top of the supporting plate (14) , preventing the wire from hanging due to the presεure of the pulp cake;

- a εecuring meanε, such as a rib (28) , placed on top of the filtrate opening for securing the wire in its place; and

- vertical supportε (15) which have been for example laεer welded to the supporting plate of the element and support the perforated plate and the wire againεt the preεεure of the pulp cake.

6. Apparatuε aε claimed in claim 3, 4, or 5, characterized in that the wire serving as the cover of the filtrate compartment (17) of the replaceable element (21) is made of polymer material, such as for example PVDF, which shrinks when heated and withstands the chemicalε used in washing and/or bleaching of fiber pulp.

7. Apparatuε as claimed in claim l or 2, characterized in that the replaceable element comprises a stiff surface having holes, slots or corresponding apertures and serving as the cover of the filter compartment;

- a supporting plate (14) guiding the wash liquid to a distribution valve (25) at the end of the drum; - vertical supports (15) which have been for example laser welded to the support plate of the element and support the the cover of the filtrate compartment against the pressure of the pulp cake.

8. The use of the apparatus aε claimed in any of the preceding claimε in washing of fiber pulp rich in silicates and/or other stickieε.

9. The use of the apparatus as claimed in any of the preceding claims in bleaching of fiber pulp rich in silicates and/or other stickieε.

10. The uεe of the apparatuε aε claimed in any of the preceding claimε in cloεed circulation wash and/or bleaching of fiber pulp.

11. A cleaning wash method of cleaning a fiber pulp washer from stickies, whereby the washer, for example a DD washer, is disconnected from normal displacement wash process, characterized in that the cleaning wash method as practiced by means of a cleaning wash apparatus comprising

- sealing elements (3, 3A) or corresponding devices required for carefully isolating the wash zones, from which filtrate produced during normal displacement wash is guided directly via a pipe line (12) for further treatment, from other process zones of the washer;

- cleaning liquid pipe line (33) which has been connected via suitable valves to the waεh zones mentioned of the washer and to their discharge pipe line (12) , and compriεeε a circulation tank (32) or a separate feed pipe line for cleaning wash liquid;

- a wash liquid pipe line (5) known per se of the washer itself and used in the displacement wash of fiber pulp, in which pipe line filtrate from stage C of a norml diplacement wash is guided to stage B and filtrate from stage B is guided directly in pipe line (12) for further treatment,-

and that the cleaning method compriseε following stage: a) the surface (7) of the drum (l) is washed with a high-pressure washing apparatus known per εe and when the first high-pressure washed portion of the filter surface (7) rotates to the zone of reduced cleaning wash, b) cleaning waεh liquid is fed from a circulation tank (32) or directly from a separate cleaning wash liquid feed pipe line in the same direction as the wash liquid of the displacement wash process onto the filter surface of the waεher drum through the pipe line/lines and wash zones from which the filtrate produced during normal displacement wash is guided directly off to further treatment, after which c) the cleaning wash liquid used in the cleaning wash of the washer is guided for further treatment either back to the circulation tank (32) for cleaning and recirculation or via a separate cleaning wash liquid dircharge pipe line to be processed elsewhere,- d) the washer drum is rotated, while disconnected from process, in stages onwards and cleaning wash liquid is supplied through the cleaning wash apparatus for the time required by the cleaning wash until the whole filter surface (7) of the drum has been cleaned; e) the washer, for example a DD washer, is reconnected to the normal displacement wash process.

12. A cleaning wash method for cleaning a fiber pulp washer from εtickieε, during which the washer, e.g. a DD washer, is disconnected from a normal displacement wash procesε, characterized in that the cleaning waεh method is practised by a cleaning wash apparatus comprising - sealing elements (3) by meanε of which the pulp discharge zone D has carefully been isolated from the previous and the following pulp treatment zoneε, viz. from the last pulp wash zone C and the feed zone A of the pulp to be washed;

- cleaning waεh liquid pipe line (33, 34, 12, 35, 36) connected by meanε of appropriate valveε to the wash zones of a DD washer from which wash liquid is guided in displacement wash to a previous wash stage, and wash liquid pipe line (5) of the washer itself with its pumping meanε (8) known per εe and uεed in the diεplacement wash of fiber pulp,-

and that the cleaning method comprises at leaεt the following stages: a) the surface (7) of the drum (1) is washed with a high-pressure washing apparatus known per se and when the filter surface has been prewashed by means of a high- pressure wash, b) cleaning wash liquid is fed from a circulation tank (32) or directly from a separate cleaning waεh liquid pipe line, in the same direction as the wash liquids of the displacement wash procesε, through an entity formed by the members used in the washer and participating in the displacement wash process, i.e. pipe lines, pumps, filter surface, perforated plates, etc. and various cleaning wash liquid pipe lines disposed in connection therewith, i. e. through a cleaning wash apparatuε, c) the cleaning wash liquid used in the cleaning wash of the washer is guided via a cleaning wash liquid pipe line to the circulation tank (32) for cleaning and recirculation or via a separate cleaning wash liquid discharge pipe line to be processed elsewhere; d) the washer drum is rotated, if required, in stages onwards and cleaning wash liquid is supplied through the cleaning wash apparatus for the time required by the cleaning wash until all the members of the displacement wash procesε, i.e. pipe lineε, pumps, filter surfaceε, perforated plateε, etc. have been cleaned; e) the washer, for example a DD washer is reconnected to the normal displacement waεh process.

13. A cleaning wash method aε claimed in claim 12, characterized in that cleaning waεh liquid iε εupplied also to the pulp feed pipe line (37, 36) of the displacement wash process.

14. A cleaning wash method aε claimed in claim 12 or 13, characterized in that the cleaning wash is carried out on the principle of closed circulation.

15. A cleaning wash method as claimed in any of claims 12, 13 or 14, characterized in that the cleaning waεh iε at firεt performed with an acid waεh liquid and then with an alkaline waεh liquid.

16. A cleaning wash method as claimed in any of claims 12, 13 or 14, characterized in that the cleaning wash is performed with an acid wash liquid.

17. A cleaning wash method of cleaning a fiber pulp washer from εtickieε, characterized in that it iε continuouε and effected in addition to the normal cleaning wash which is performed at a preεsure of about 800 - 1200 kPa (8 - 12 bar) after the washed fiber pulp has been detached while the procesε is running,- and that it comprises - one nozzle or several adjacent nozzleε movable in the axial direction of the waεher drum and disposed separately following the normal cleaning wash zone of the washer, i.e. after the space where the wash nozzles are located which are used in connection with the wash following the detachment of pulp cake in a normal displacement wash stage of fiber pulp,- appropiate sealing elements for isolating the cleaning wash zone from the pulp to be washed and from the discharge zone of the washed pulp/the normal waεh zone,-

- cleaning wash liquid pipe line with the required tanks and pumps, or a separate feed and return pipe line;

and that

- cleaning wash liquid is supplied onto the filter surface of the drum on the area defined by the operation range of the nozzles while the nozzles are moved continuously in the axial direction of the drum whereby a spiral area of the drum shell is continuouεly cleaned the width of which is determined by the coverage of the high- pressure nozzles, or in stages whereby a zone of the drum shell is cleaned the width of which is defined by the coverage of the high-pressure nozzles;

- the wash liquid used in the cleaning wash of the washer is returned to the circulation tank (32) for cleaning and for reuse or it iε pumped/guided to be processed somewhere else or the wash liquid used in the cleaning wash of the washer is guided via a separate discharge pipe line to be processed somewhere else.

18. A cleaning wash method as claimed in claim 17, characterized in that the cleaning wash is carried out on the principle of closed circulation.

19. A cleaning wash method as claimed in claim 17 or 18, characterized in that the cleaning wash is at first performed with an acid wash liquid and then with an alkaline wash liquid.

20. A cleaning waεh method of cleaning a fiber pulp waεher from εtickieε, characterized in that it continuous and is effected in addition to the normal cleaning wash which is performed and at a pressure of about 800 - 1200 kPa (8 - 12 bar) after the washed fiber pulp has been detached while the process is running,- and comprises apparatus in which there are

- adjacent high-pressure nozzles disposed in the axial direction along the whole length of the drum and separately following the normal cleaning wash zone of the washer, i.e. after the space where the wash nozzles are located which are used in connection with the wash following the detachment of pulp cake in a normal displacement wash stage of fiber pulp;

- appropriate sealing elements for isolating the cleaning wash zone from the feed zone A of the pulp to be washed and from the discharge zone of the washed pulp/the normal wash zone D; - cleaning wash liquid pipe line with the required tanks and pumpε or εeparate εupply and return pipe lines;

and that

- high-pressure cleaning wash liquid iε supplied onto the filter surface of the drum on the area defined by the operation range of the high-pressure nozzles,-

- the wash liquid used in the cleaning wash of the washer is returned to the circulation tank for cleaning and for reuse or it iε pumped/guided to be processed somewhere else or the wash liquid used in the cleaning wash of the washer is guided via a separate discharge pipe line to be processed somewhere else.

21. A cleaning wash method as claimed in claim 20, characterized in that the cleaning wash iε carried out on the principle of cloεed circulation.

22. A cleaning waεh method as claimed in claim 20 or 21, characterized in that the cleaning wash is at first performed with an acid wash liquid and then with an alkaline wash liquid.

23. The use of the method as claimed in any of claims 17 - 22 for washing fiber pulp rich in silicates and/or other stickies.

24. The use of the method as claimed in any of claims 17 - 22 for bleaching fiber pulp rich in silicates and/or other stickieε.