SE1950797A1 - Disc filter for lime mud slurry - Google Patents

Abstract

The invention relates to a disc filter (1) for separating lime mud particles comprised in lime mud slurry from liquid. In particular, the invention is directed to prevent the buildup of lime mud deposits in the disc filter (1). The disc filter (1) comprises one or several filter disc units (4) unit arranged on a rotatable shaft (3). The filter disc unit (4) comprises at least a first filter disc (4b) and a hollow interior (4a). The hollow interior (4a) is in communication with a channel (3a) in the rotatable shaft. The filter disc unit is partly immersed in a vessel (2) comprising lime mud slurry. The lime mud slurry is forced through the filter disc (4b) by a pressure difference. When the lime mud slurry passes through the filter disc, lime mud particles accumulate on the surface of the filter disc (4) while the filtered liquid fraction is guided away from the disc filter via the channel (3a) in the shaft (3). The disc filter further comprises a scraper (12) arranged to scrape off lime mud accumulated on the filter disc (4b) to be collected by a mud chute (10). In order to prevent buildup of deposits of lime mud particles on the mud chute walls (10a, 10b) there is a protective screen (32) arranged between the mud chute walls (10a, 10b) such that at least a part of the lime mud particles scraped off from the filter disc (4b) are prevented from reaching a mud chute wall (10a, 10b).

Description

DISC FILTER FOR LIME MUD SLURRY TECHNICAL FIELD The invention relates to a disc filter for separating lime mud particles comprised in lime mud slurry from liquid.

BACKGROUND ln the recovery process of a pulp mill, green liquor is the diluted and dissolved smeltof sodium carbonate, sodium sulfide and other compounds from the recovery boiler.The sodium carbonate in the green liquor may be re-causticized to sodium hydroxideby adding burnt lime (CaO) as a step towards regenerating white liquor used in thepulping process. The re-causticizing process generates lime mud as fine particles ofcalcium carbonate (CaCOg) which are separated from the liquor, washed anddewatered before circulated to the lime kiln where the CaCOß is transformed back toCaO. Several separation stages are used in the processing of green liquor and white liquor, involving separation units ofdifferent kinds, including disc filters.

Filters used for separating the solid calcium carbonate particles from the liquid is forexample disclosed in WO 2017 / 082 803. ln WO 2017 / 082 803 is a disc filtercomprising a multitude of filter discs disclosed. The filter disc comprises a multitudeof perforated discs provided with filter cloths covering the perforated discs. A liquid orslurry comprising particulate matter is forced through the perforated discs as the discfilter disc is rotating in a container filled with the liquid or slurry to be filtered. lngeneral, the level of the slurry is reaching up to the middle of the discs such thatapproximately one half of the disc filter is immersed in the slurry to be filtered. As thefilter rotates, and liquid is forced through the discs, solid matters are accumulated onthe surface of the discs and build up a layer of solids on the surface. ln general, athin layer of solids close to the filter is desired for forming a so called pre coat layerwhich functions as a filter media in addition to the filter cloth. However, the pre coatlayer is not allowed to increase beyond a certain thickness why excess solid matter isscraped off the disc by a scraper, e.g. a doctor blade, and allowed to fall down into achute arranged below the doctor blade. Similar arrangements disclosing disc filters are also disclosed in WO 2017/007 417, WO 2014/011089 and WO 2018/186795 inwhich solid matter is scraped off and falls into lime mud chutes arranged betweenand on each side of the filter discs. lt has been discovered that deposits of lime mud form on the inside surfaces of theselime mud chutes, e.g. when used for the purpose as disclosed in WO 2017 / 082 803.These deposits grow over time and eventually prevent lime mud from falling throughthe lime mud chute. The deposits could lead to overfilling of the chute that may causemechanical damage to the filter discs. ln order to avoid this damage it is necessary tohave regular inspections when deposits are removed. This is costly and deposits notare removed, the damages cause production stops and involves costs associatedtherewith.

OBJECT OF THE INVENTION lt is an object of the invention to provide a disc filter for lime mud dewatering whereinless or no such deposits form on the inside surfaces of the lime mud chutes.

SUMMARY The invention relates to a disc filter used for filtering of solid particles from lime mud slurry in the recovery process of a pulp mill.

The invention is in particular directed to a device and method which reduces theaccumulation of lime mud to mud chute walls in the disc filter by the use of aprotective screen. The term "screen", in this context, is a member that preventsscraped off lime mud from reaching a wall arranged behind said screen. The screenis generally defined to have a height and a perpendicular width defining a planeforming the main surface area of the screen. The screen also has a thickness in adirection perpendicular to the plane defined by the height and width. The thickness ofthe screen is generally considerably less than its height and width. The height is ingeneral longer than its width but depending on the design of the disc filter, the widthof the screen could be longer than its height. The screen is for example a sheet or acurtain being essentially impermeable or a net or perforated plate. Furthermore, the screen could be made from a multitude of individual elements, e.g. several elongatedelements such as plastic or rubber strips or metal chains, hanging down so astogether form a screen protecting a wall from being hit by the lime mud. ln general,when in rest position, the screen extends within an essentially vertical plane having aheight in its vertical direction, a width in the horizontal direction and a thickness in thedirection perpendicular to its height and width directions. The screen may forexample be arranged this way by being attached at its upper edge and allowed tofreely hang down therefrom so as to be essentially parallel to and cover a verticalwall to be protected by the screen. ln case the screen is made from a sheet havingtwo main surfaces, the protective screen may thus be arranged such that that one ofits surfaces faces and is essentially parallel to a wall it is protecting.

The protective screen comprises a first surface that faces said first wall and isarranged to intercept lime mud scraped off from the filter and prevent lime mud fromreaching and being stuck on the second wall. Hence, the lime mud falls into the limemud chute over said first mud chute wall, either directly or via the protective screen The protective screen may for example be a flexible screen. The term "flexible", inthis context, means something being capable of bending and deflecting. By "flexibleelement" is meant to include an element which comprises at least one portion beingcapable of bending and deflecting, i.e. it is not necessary that the entire flexibleelement is capable of bending and deflecting. A flexible element is for example asheet made from rubber or other elastic material being able to be bent or a multitudeof rigid elements being joined by a pivotal connection. As will be better understood inthe following description, a flexible element as disclosed in the present invention ismeant to include an element which renders the screen capable of bending ordeflecting when subjected to an impact force from a substance, e.g. lime mud, hittingthe screen or gravity forces from a substance adhered to the screen. As opposed toa flexible screen, the screen could also be a rigid structure, e.g. a plate made of apolymer or metal, which essentially not is deformed by the forces from a substancehitting or adhering to the screen. Hence, there is a multitude of different ways to design and arrange a screen in the disc filter so as to reduce the amount of lime mud hitting and being adhered to the walls.

The object of the invention is achieved with a disc filter as defined in independentclaim 1 and a method as disc|osed in independent claim 12.

Independent claim 1 relates to a disc filter for filtering lime mud slurry containing limemud particles. The disc filter comprises a vessel including a chamber configured toreceive said lime mud slurry and a rotatable shaft that extends through said chamberand comprises a channel that extends along a longitudinal axis of said shaft. Thedisc filter further comprises at least one filter disc unit arranged on said shaft, whichfilter disc unit comprises at least one filter disc and a hollow interior. The hollowinterior is in communication with said channel and in communication with saidchamber via said filter disc. The disc filter is also provided with means for creating apressure difference between the chamber and the hollow interior of the filter disc unitin order to force said lime mud slurry through the filter disc into the hollow interior. lngeneral, a filter disc unit comprises two filter discs being concentrically spaced apartfrom each other and joined together with a cylindrical wall along their respectivecircumferences which together with the filter discs define a hollow interior eventhough it may be possible having only one filter dis in the filter disc unit. The disc filtercomprises at least one scraper arranged to scrape off lime mud from the surface ofthe filter disc. The disc filter also comprises at least one mud chute arranged tocollect lime mud being scraped off the filter disc, which mud chute comprises a firstmud chute wall proximal to said filter disc and an opposite second mud chute wall.The disc filter according to the invention further comprises a protective screen atleast partly arranged between said first and second mud chute walls. The protectivescreen is arranged to intercept lime mud scraped off from the filter and prevent limemud from reaching and being stuck on the second wall. Hence, the lime mud fallsinto the lime mud chute over said first mud chute wall, either directly or via theprotective screen.

The protective screen thus prevents lime mud that falls over the first wall and into thelime mud chute from reaching the second wall. ln the absence of said screen, at leastsome of the lime mud particles that fall into the lime mud chute would follow a trajectory that eventually brings said lime mud particles into contact with the secondmud chute wall, where some of the particles become stuck and form a deposit thatgrows over time. That is, buildup of lime mud on the second mud chute wall isprevented by the screen.

An advantage by using a protective screen in front of a mud chute wall is that thescreen may more easily be designed to prevent lime mud from being stuck to andaccumulated on its surface than the surface of a wall. For example, the screen couldbe made from a material having properties preventing the lime mud to stick to thesurface which not is suitable to be used as the material for the wall construction orhaving certain desired mechanical properties inherent in the selected material to beused in the screen. ln addition, a separate screen could also be removed in order tobe cleaned or replaced.

According to one embodiment of the invention, the screen comprises a flexibleelement so as to form a flexible screen. The flexible screen could for example bemade from a sheet of material which is flexible by itself, e.g. rubber, such that thescreen is a flexible sheet. Alternatively, the flexible screen could be made from anumber of rigid and stiff parts which are pivotally or bendably connected such thatthe overall structure of the screen is flexible. The flexibility of the screen reduces therisk that lime mud particles become stuck to the flexible screen. Also, the flexibility ofthe screen enables that lime mud deposit formed and accumulated on the flexiblescreen may cause bending or deformation of the flexible screen. The deformation orbending of the screen causes bending forces or stress between the sheet and thedeposits accumulated on the sheet thus reducing the adherence of the lime mud tothe flexible screen such that the lime mud comes loose and falls through the limemud chute. A flexible screen could be used to provide a screen which retains its functionality over long periods of time with little or no maintenance. ln case the protective screen is made from a sheet material, the sheet material couldbe either flexible or rigid, e.g. a rubber mat or steel plate. However, in either case, theflexible screen could be prepared to have a non-sticky surface preventing the limemud from adhering to its surface and facilitating the lime mud to detach from theflexible screen. A non-sticky surface could be achieved by using a non-sticky material throughout a sheet or by applying a non-sticky coating such as Teflon ® onto thesurface. Alternatively, the surface structure of the screen could also be modified, e.g.by having irregularities or corrugations forming channels enhancing detachment ofdeposited material. ln case the screen is a sheet, the sheet may be arranged suchthat a first surface that faces the first wall is arranged to intercept lime mud scrapedoff from the filter and prevent lime mud from reaching and being stuck on the second wall.

The protective screen could be arranged at a distance from the first and second mudchute walls such that a certain degree of movement of the screen relative the walls isfacilitated. The screen may for example swing, vibrate or, in the case of a flexiblesheet, bend and deform which actions may aid in reducing the adherence of limemud deposits on the screen, ln particular, if a flexible screen is arranged at adistance from the second wall, any deposit formed and accumulated on the flexiblescreen may eventually bend the flexible screen backwards towards the second wall,thus reducing the adherence of the deposited lime mud to the screen and facilitatingthe lime mud to detach and fall down. ln case there is a desire that the screen shouldbe able to move, the screen should be arranged at a sufficient distance from thewalls for allowing a desired movement to occur for achieving the desired effects. lngeneral, a distance of about 5 cm between the protected wall and the screen may besuitably used even though a shorter, or longer, distance could be used. For example,there will generally be a positive effect from an increased freedom of movement for adistance as short as 10 mm, or even 5 mm, between the screen and the closest wallas compared to a screen being arranged adjacent to and in contact with the wall.When the screen is intended to only protect one wall, i.e. the lime mud chute isscraped off from only one filter closer to a first mud chute wall, the screen is generallylocated closer to the second, opposite mud chute wall in order to intercept lime mudparticles that falls over the first wall and into the lime mud chute.

The disc filter may comprise an oscillation generating means for generating recurrentor intermittent oscillations in said protective screen. These oscillations, or vibrations,cause the lime mud to come loose from the protective screen and fall through thelime mud chute. Alternatively, or in addition to an oscillator, the disc filter maycomprise a shape-changing means for recurrently or intermittently changing the shape of the flexible screen. Alterations of the shape of the flexible screen cause thelime mud attached thereto to come loose and fall through the lime mud chute. Saidshape-changing means may, for example, comprise a mechanism for supplying afluid, e.g. air or any other suitable gas, into an inflatable part of the flexible screen. Ofcourse, said mechanism may also be configured to remove said fluid from the inflatable part of the flexible screen.

The protective screen is particularly advantageous when the lime mud chute is fedfrom one side only. Lime mud chutes arranged between two filter discs and arrangedto receive lime mud from both filter discs do not have the same problem, as the twoflows of lime mud will intersect at the center of the lime mud chute and thus preventone another from reaching the walls of the lime mud chute. Thus, in someembodiments, only lime mud chutes fed from one side only are provided with flexiblescreens. ln case a screen is desired for intercepting lime mud from two sides, thescreen is preferably located close to the middle.

Advantageously, the protective screen extends in parallel with the walls of the limemud chute. The flexible screen preferably has a vertical extension such that itextends into the lime mud chute and intercepts essentially all, or at last the vastmajority, of lime mud which should have reached the wall unless the protectivescreen were present. Likewise, the screen is preferably designed to have a widthextending laterally such that no or insignificant amounts of lime mud may pass by itslateral sides and reach the wall. Generally, the screen extends essentially from onelateral side to the other covering essentially the entire width of the second wall of the lime mud chute.

As briefly discussed above, the screen may comprise a fixed upper portion and amovable lower portion. That is, the upper portion is fixed whereas the lower portion ismovable towards and away from the first and second walls of the lime mud chute.For example, the fixed upper portion may be connected to the lime mud chute by afastening means extending across the upper inlet of the lime mud chute along thewidth of the screen.

The invention also relates to a method for reducing deposits of lime mud in a discfilter for filtering lime mud slurry. The disc filter may be designed as described above.According to the method, the disc filter used comprises one or several filter disc unitsmounted onto a rotatable shaft that extends through a chamber configured to receivelime mud slurry. There is a channel that extends along a longitudinal axis of the shaftand the filter disc units comprises a first filter disc separating a hollow interior in thefilter disc unit in communication with said channel from an outer chamber. Themethod comprises the steps of: - forcing said lime mud slurry from said outer chamber via said filter discinto the hollow interior of the filter disc whereby lime mud will be accumulated on thesurface of the filter disc - scraping off lime mud particles accumulated on the surface of the filterdisc to be directed into an upper inlet of a mud chute over a first mud chute wallwhereby at least some of the lime mud is directed towards an opposite second mudchute wall, and- - Placing a screen in the disc filter so as to extend into a void space ofthe mud chute and arranging the screen to intercept at least a part of the lime mudparticles scraped off from the filter disc such that lime mud particles are prevented bythe screen from reaching the second mud chute wall.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will now be described in more detail withreference to the appended drawings, wherein some parts have been removed for thesake of clarity, and wherein: figure 1 is a schematic illustration of a disc filter according to a firstembodiment of the invention; figure 2 is a cross-sectional view along line AA in figure 1; figure 3 is a cross-sectional view through a lime mud chute according toa first embodiment of the invention; figure 4 is a cross-sectional view through a lime mud chute according toa first embodiment of the invention; figure 5 is a cross-section view through a lime mud chute according to a second embodiment of the invention; and figure 6 is a cross-section view through a lime mud chute according to a third embodiment of the invention.

DETAILED DESCRIPTION The invention will now be described in detail with reference to figures 1-4, whereinlike features are represented by like reference numerals.

Figure 1 shows a disc filter 1 according to a first embodiment of the invention. Thedisc filter 1 is used to filter out fine lime mud particles in lime mud slurry. The lime mud slurry may for example mainly comprise lime mud and weak liquor.

The disc filter 1 comprises a vessel 2, which comprises a chamber 6 arranged to befed with said lime mud slurry. The lime mud slurry can be fed to the vessel 2 by amultitude of nozzles (not shown) distributed over the length of the vessel 2.

A rotatable shaft 3 with a plurality of filter disc units 4 arranged on it extends throughthe chamber 6. The rotatable shaft 3 is arranged to be rotated around a longitudinalaxis X by means of a motor 5 and comprises an internal channel 3a that extendsalong the longitudinal axis X.

The filter disc units 4 are arranged to rotate with the rotatable shaft 3. Each filter discunit 4 comprises a hollow interior 4a located between two filter discs 4b. A filter disc4b may comprise a perforated material, such as metal, covered with a fine meshcloth, such as a polypropylene cloth. The hollow interior 4a is divided into radiallyextending sectors (not shown) by means of radially extending sector walls (notshown). The hollow interior 4a is in fluid communication with the inner channel 3a inthe rotatable shaft 3 and with the interior of the chamber 6 by means of said filterdiscs 4b. Lime mud particles are removed from the lime mud slurry by forcing theweak liquor through the filter disc 4b, by creating an under pressure within the hollowinterior 4a of each filter disc unit 4, while lime mud particles accumulate on the filterdiscs 4b.

The disc filter 1 further comprises a plurality of essentially vertical lime mud chutes10 arranged so that each filter disc unit 4 is arranged between a pair of lime mudchutes 10. Each lime mud chute 10 is configured to receive scraped off lime mudfrom the filter discs 4b of the adjacent filter disc units 4. The lime mud is scraped offthe filter discs 4b by means of scrapers 12 arranged at upper inlets of the lime mudchutes 10 and falls through the lime mud chutes 10 to a lime mud conveyor 11arranged below the lime mud chutes 10. The lime mud conveyor 11 is configured toconvey the lime mud to further processing equipment (not shown). A precoat or heelcake of lime mud (not shown in figure 1) is established on the filter discs 4b byarranging an edge of each scraper 12 at a distance from a surface of thecorresponding filter discs 4b.

The internal channel 3a is also connected to a weak liquor separator 7, wherein weakliquor and gas from the filter discs 4 are separated from one another. The weak liquorseparator 7 has a tangential inlet that forces the weak liquor flow into a vortex spiralon the wall of the separator. Gas accumulates in the center of the weak liquorseparator 7 and is withdrawn at the top and transported to a spray condenser 8 andfrom there to a vacuum pump 9, whereas weak liquor is withdrawn at the bottom ofthe separator 7. The spray condenser 8 condenses part of the water in the gas fromthe weak liquor separator 7 to increase the efficiency of the vacuum pump 9.

The chamber 6 is filled to a certain level with said lime mud slurry, in thisembodiment to just below the rotatable shaft 3. Each filter disc unit 4 is rotated (thedirection of rotation being indicated with ROT, see figure 2) by means of the rotatableshaft 3, so that each sector of the filter disc unit 4 in turn is submerged in the limemud slurry. The vacuum pump 9 creates an under pressure within the hollow interior4a of the filter disc unit 4 and due to the under pressure, the weak liquor portion ofthe lime mud slurry is forced through the filter disc 4b while lime mud particlesaccumulate on the surface of the filter disc 4b. The weak liquor is then transported tothe weak liquor separator 7, whereas lime mud is scraped of the filter disc 4 bymeans of a scraper 12 and falls through a lime mud chute 10 to the lime mud conveyor11. 11 Now referring to figure 4, the filter disc 4b itself may not be sufficient to reach lowparticle content in the filtered weak liquor. The filter disc 4b is rather used to enableand facilitate the forming of a precoat PC layer of fine particles on the filter disc 4b,also referred to as heel cake, and this precoat PC is used to filter the lime mud slurry.That is, effective filtration can begin once a precoat PC with sufficient thickness hasbeen established on surface of the filter disc 4b. The scraper 12 comprises a doctorblade 12a, and the precoat PC is created by arranging the doctor blade 12a at adistance from the surface of the filter disc 4b. As each filter disc unit 4 sector passesthrough the lime mud slurry, additional lime mud particles are deposited on theprecoat PC layer. As the sectors pass the doctor blade 12a (see figure 2), the limemud layer formed on top of the precoat is removed by the doctor blade 12a. Theremoved lime mud falls by action of gravity into the lime mud chute 10 arrangedimmediately below the scraper 12. Conventionally, first a precoat PC is establishedwith a starting thickness of about 1.0-3.0 cm, and thereafter additional layers of limemud particles on top of the precoat PC are scraped off by the doctor blade 10b.

As filtering proceeds, the outermost layer of the heel cake/precoat PC will be blockedby smaller lime mud particles. To overcome this problem, the doctor blade may beadvanced towards the surface of the filter disc 4b at a small incremental adjustmentbeing a fraction of the starting precoat PC thickness to scrape of the outer layer ofthe precoat PC that has become blocked. When the original precoat PC finally has been consumed, an entirely new precoat is formed.

The disc filter 1 may also comprise wash showers and a precoat renewal highpressure jet system, for example the SQUIRTW' system sold by Valmet. ln figure 2 isdisclosed how a SQUIRTW' system can include a high-pressure nozzle 21, whichslowly can be moved radially on a rail 20, and this high-pressure jet can dislodge astrip of precoat from the filter disc 4b as well as provide for a thorough filter clothcleaning. One advantage with the SQUIRTW' system is that the entire precoat can berenewed, as it becomes blocked over time, and the filtering process does not need tobe halted for precoat renewal. Alternatively, the high-pressure nozzle 21 may belocated on a swan neck protruding downwards into the lime mud slurry. The high-pressure jet may dislodge a strip of precoat from the filter disc as well as provide fora thorough filter cloth cleaning below the liquor level. The advantage here is that the 12 dislodged strip quickly becomes refilled, at least in part by new unblocked lime mud particles before this strip ascends from the lime mud slurry.

A problem may arise when a lime mud chute is fed with scraped off lime mud, inparticular when lime mud is fed from one side only. What happens is that the scrapedoff lime mud may come into contact with an inner surface on the opposite side of thelime mud chute and adhere thereto. This deposit grows over time and eventuallyforms an obstacle that prevents lime mud from falling through the lime mud chute.This problem is more likely to arise when lime mud is fed from one side only since iflime mud is fed from two sides, a large portion of the lime mud scraped off will collidesuch that lime mud from each side will prevent the lime mud from the other side tofulfill its trajectory towards an inner surface on the opposite side of the lime mudchute.

A solution to this problem is shown in figure 3, which shows in detail the left-mostlime mud chute 10 in figure 1.

The lime mud chute 10 comprises a first mud chute wall 10a proximal to said filterdisc 4 and an opposite second mud chute wall 10b, which first and second mudchute walls 10a,b are connected by side walls (not shown) and form between them aspace 10c with an upper inlet 10d and a lower outlet 10e.

A scraper 12 comprising a doctor blade 12a attached to a framework 12b is mountedon the lime mud chute 10 above the first wall 10a. The doctor blade 12a is configuredto scrape off excessive lime mud particles 31 from the precoat PC layer on thesurface of the filter disc 4a. These lime mud particles 31 fall through the upper inlet 10d and through the space 10c towards the lower outlet 10e. ln order to avoid build of lime mud deposits on the mud chute wall, the disc filter 1further comprises a protective screen 32. The protective screen extends verticallythrough a portion of the space 10c at a distance from the first and second walls 10a,b. The screen 32 is attached to the lime mud chute 10 by any suitable means and isherein exemplified by a fastening means 33 that is attached to and extends between 13 the side walls of the lime mud chute 10. The screen 32 is arranged to intercept limemud particles 31 that fall through the space 10c.

The screen 32 may be designed to be flexible. ln particular, it is desired to design thescreen 32 to be bendable at least in a direction towards the second mud chute wall10b. The use of a screen 32 being flexible reduces the risk that the lime mudparticles 31 will attach to said screen 32 as they come into contact with it. lnstead,the lime mud particles 31 will have a higher tendency to reflect off a flexible screenand fall through the lower outlet 10e. Should lime mud particles 31 attach to a flexiblescreen, then the flexible screen will have an increased tendency to bend towards thesecond wall 10b of the lime mud chute 10 (see figure 4) as lime mud particles 31accumulate on the screen 32. The increased tendency to bend the screen is due toan increased weight, and thus increased gravity forces, from accumulated lime mudattached to the screen 32. As a consequence of the bending action, the adhesiveforces between the lime mud and the screen 32 will be reduced and the lime mudparticles 31 will come loose and fall through the space 10c to the lower outlet 10e. lnorder to allow a screen 32 to bend towards the second wall 10b, the screen 32 ispreferably arranged at such a distance from the second wall that the screen 32 maybend without being prevented to deform by the second wall 10b since the bending isintended to reduce the adhesive forces between the screen 32 and lime mudparticles 31 accumulated onto the screen 32. ln addition, a flexible screen maydeform when hit by lime mud being scraped off such that lime mud previouslyaccumulated on the flexible screen may detach due to vibrations or swinging of thescreen arising from the lime mud hitting it.

Advantageously, the screen 32 comprises an outer layer 32a facing the first mudchute wall 10a, which outer layer 32a inhibits build-up of deposits on the protectivescreen 32. The outer layer 32a may comprise an anti-fouling material such aspolytetrafluorethylene or any other suitable material reducing the stickiness of thesurface. Alternatively, or in addition, the structure of the surface could be modified, e.g. by having wedge-shaped elements pointing upwards or vertical channels.

The fastening member 33 schematically shown in figure 3 may be any suitable typeof fastening member, e.g. a clamping strip. However, it is not necessary that the 14 screen 32 is attached to the lime mud chute 10, as it may also be attached to another part of the disc filter 1 and extend into the space 10c in the lime mud chute 10.

Figure 5 shows an additional feature which may be added to the above describedembodiments of the invention, wherein the fastening member 33 also functions as anoscillation generating means configured to generate recurrent or intermittentoscillations in said screen 32, i.e. to vibrate said screen 32. These vibrations causelime mud particles 31 adhered to the screen 32 to come loose and fall through thelime mud chute 10. ln case an oscillation generating means is used it may be anadvantage to use a rigid plate as a screen 32 since oscillations, in particular highfrequency oscillations, may be dampened by a flexible sheet while such vibrationseasier propagate in a rigid plate. For a flexible screen it may be a better alternative touse vibrations of lower frequency and higher amplitude, e.g. having an oscillator causing the flexible screen to move as a rolling wave.

Figure 6 shows yet another alternative embodiment of the invention, wherein thescreen 32 comprises an inflatable part 32a and wherein the disc filter 1 comprises ashape-changing means 34 comprising a mechanism for, recurrently or intermittently,supplying a fluid, e.g. air, into said inflatable part 32a. Lime mud particles 31 stuck tothe flexible screen 32 will fall off the flexible screen 32 when it is inflated to inducebending and/or shear forces working to reduce the adhesive forces between the screen 32 and the accumulated lime mud particles 31 adhered thereto.

Now returning to figure 1, is shown a disc filter 1 wherein only the outermost limemud chutes 10 are provided with protective screens 32. The reason for this is that theproblem of lime mud particles forming deposits on the lime mud chute walls 10a, 10b is primarily associated with lime mud chutes 10 that are fed from one side only.

However, it is possible to arrange screens 32 in the other lime mud chutes as well. lnthat case, the protective screen 32 is preferably arranged in an arrangement similarto the one described above but preferably adjusted concerning its distance to the firstand second mud chute walls 10a,b such that the screen is equidistant from the mudchute walls 10a, b.

The screen 32 may be flexible, rigid or a combination thereof. For example, the flexible screen 32 may comprise a fixed upper portion and a flexible lower portion. lt is to be understood that the present invention has been described by way ofillustration and not limitation. The invention may be modified in a number of ways.The disc filter may, for example, comprise a compressor arranged to injectpressurized gas into the chamber in the disc filter to force the liquor through the filterdisc and into the filter disc units. lt is also possible to implement the above describedsolution in any part of the recovery area of a pulp mill, e.g. in a first stage of a two-stage pressurized white liquor disc filter and/or in a pressurized lime mud disc filter ofone or two stage design, where residual alkali is washed out from the lime mudobtained from white liquor filters preceding the lime mud wash, and located ahead ofthe lime kiln. lt is also possible to combine the above described embodiments in anysuitable way, e.g. by providing a flexible sheet that is both inflatable and arranged to oscillate.

Claims (12)

1. Disc filter (1) for lime mud slurry containing lime mud particles, which disc filter (1)comprises: - a vessel (2) comprising a Chamber (6) configured to receive said limemud slurry; - a rotatable shaft (3) that extends through said chamber (6) andcomprises a channel (3a) that extends along a longitudinal axis (X) of said shaft (3); - at least one filter disc unit (4) unit arranged on said shaft (3), which filterdisc unit (4) comprises at least a first filter disc (4b) and a hollow interior (4a), whichhollow interior (4a) is in communication with said channel (3a) and also incommunication with said chamber (6) via said first filter disc (4b); - means for creating a pressure difference between the chamber (6) andthe hollow interior (4a) of the filter disc unit (4) in order to force said lime mud slurrythrough said first filter disc (4b) into the hollow interior (4a); - at least one scraper (12) arranged to scrape off lime mud from said firstfilter disc (4b); - at least one lime mud chute (10) arranged to collect lime mud beingscraped off the first filter disc (4b) and entering the lime mud chute (10) via an upperinlet (10d), which lime mud chute (10) comprises a first mud chute wall (10a)proximal to said first filter disc (4b) of the filter disc unit (4) and an opposite secondmud chute wall (10b); characterized in thatsaid disc filter (1) comprises a protective screen (32) at least partly arranged betweensaid first and second mud chute walls (10a, 10b), which screen (32) is arranged tointercept lime mud that falls into the lime mud chute (10) over said first mud chutewall (10a) such that at least a part of the lime mud particles scraped off from the firstfilter disc (4b) are prevented from reaching the second mud chute wall (10b).

2. Disc filter (1) according to claim 1 characterized in that said screen (32) comprises a flexible element so as to form a flexible screen.

3. Disc filter (1) according to any of claims 1 or 2 characterized in that said screen(32) is made of a sheet material. 17

4. Disc filter (1) according to claim 3 characterized in that said screen (32) is madeof rubber or a plastic polymer.

5. Disc filter (1) according to any previous claim characterized in that said screen(32) is arranged to be spaced apart from said first and second mud chute walls (10a,10b) when said screen (32) is at rest.

6. Disc filter (1) according to claim 5, wherein said screen (32) is located at adistance from the second mud chute wall (10a) being at least 5 mm, preferably atleast 10 mm, up to a distance being half the distance between the first mud chutewall (10a) and the second mud chute wall (10b).

7. Disc filter (1) according to any previous claim, wherein an upper portion of saidscreen (32) is connected to said lime mud chute (10) by a fastening means extendingacross the upper inlet (10d) of said lime mud chute (10).

8. Disc filter (1) according to any of the preceding claims, wherein said screen (32) ismade of a non-sticky material or having its surface coated by a non-stickycomposition such as polytetrafluorethylene.

9. Disc filter (1) according to any of the preceding claims, wherein said disc filter (1)comprises a vibration generating means arranged to generate vibrations in said sheet.

10. Disc filter (1) according to any of the preceding claims, wherein said flexiblescreen (32) comprises an inflatable part and wherein said disc filter (1) comprises amechanism configured to inject a fluid into said inflatable part.

11. Disc filter (1) according to any of the preceding claims, wherein said disc filter (1)comprises a washing means configured to apply a washing fluid to the surface ofsaid screen (32). 18

12. Method for reducing deposits of lime mud in a disc filter (1) for filtering lime mudslurry, said disc filter (1) comprising one or several filter disc units (4) mounted onto arotatable shaft (3) that extends through a chamber (6) configured to receive said limemud slurry and comprises a channel that extends along a longitudinal axis of saidshaft (3), said filter disc units (4) comprising at least a first filter disc (4b) separating ahollow interior in communication with said channel from said outer chamber (6)wherein said method comprises the step of: - forcing said lime mud slurry from said outer chamber (6) via said filterdisc (4b) into the hollow interior of the filter disc unit (4) whereby lime mud will beaccumulated on the surface of the filter disc (4b) - scraping off lime mud particles accumulated on the surface of the filterdisc (4b) to be directed into an upper inlet (10d) of a mud chute (10) over a first mudchute wall (10a) whereby at least some of the lime mud is directed towards anopposite second mud chute wall (10b) characterized in- - Placing a screen (32) in the disc filter (1) so as to extend into a voidspace of the mud chute (10) and arranging the screen (32) to intercept at least a partof the lime mud particles scraped off from the filter disc (4b) such that lime mudparticles are prevented by the screen (32) from reaching the second mud chute wall(10b).