SE538514C2 - Method for producing a sludge mixture in a pulp mill - Google Patents

Abstract

ABSTRACT Method of producing a sludge mixture (14) in a pulp mill, Wherein said methodcomprises the steps of supplying a size reducing mill (6) With an undersized fibrousmaterial (5) comprising pin chips, shiVes, f1nes and other undersized material from aWood chip classifier (2), comminuting said undersized material (5) in said size reducingmill (6), suspending the resulting comminuted fibrous material in a Waste Water floW (8)thereby achieving a aqueous fibrous mixture (ll), and mixing said aqueous fibrous mixture (ll) With a biosludge to form a sludge mixture (l4). A device for producing a sludge mixture (14) in a pulp mill, said device comprises asize reducing mill (6) arranged to comminute an undersized fibrous material (5)comprising pin chips, shiVes, f1nes and other undersized material from a Wood chipclassifier (2) a suspension tank (7) arranged to suspend the resulting comminutedfibrous material in a Waste Water floW (8) thereby achieving a fibrous mixture, and amixing tank (12) arranged to mix said fibrous mixture With a bio sludge to form a sludgemixture (l4).

Description

A METHOD AND DEVICE FOR PRODUCING A SLUDGE MIXTURE IN A PULPMILL TECHNICAL FIELDThe present invention relates to a method of producing a sludge mixture in a pulp mill and a device for producing a sludge mixture in a pulp mill.

BACKGROUND ART In a typical pulp making process, Wood logs are debarked and chipped, and individualcellulose f1bers are then freed or liberated from the chips in subsequent treatmentsbefore ultimate paper or paperboard Web formation. The debarking and chipping stepsare carried out in What usually is called a Wood room, i.e. an installation for the makingof chips, norrnally including equipment for cross-cutting, debarking, chipping and chipscreening. A common Way to liberate the cellulose f1bers in the subsequent treatments isby cooking the Wood chips With chemicals at elevated temperatures and pressures indigesters to remove lignin from the chips, Which holds the f1bers together. For thesubsequent paper or paperboard making process, it is desirable that the delignified f1bers obtained exhibit substantially similar characteristics.

To minimize the production of undercooked or overcooked chips in the digester, it isnecessary that the cooking liquor penetration into the chips is substantially similar forall chips, so that the effects of temperature, pressure, and time are similar for all chips.Therefore, it has been found desirable in the past to utilize chip screening apparatus,Which removes both undersized and oversized chips, so that the undersized chips,including pin chips, shives, saWdust, and f1nes, can be treated separately and theoversized chips can be passed through chip size reducing apparatus, e. g. a slicer, prior todigesting. The slicer reduces the oversized chips to a desired size, but it also creates additional undersized chips, pin chips, shives, saWdust, and f1nes.

It is knoWn to use the undersized material leaving the chip screens or classif1ers bysupplying it as additional fuel in a boiler in the pulp mill, usually after pulverizing the undersized material.

Further, during the treatment of the Wood in the Wood room, large amounts of Water areused. To avoid pollution of the environment, the effluent from the Wood room comprising solid matter and sludge, has to be treated so that the Water can be recirculated to the Wood room, While the sludge after deWatering can be supplied to the boiler and bumed to provide heat energy.

US 5,l37,62l (Brown) disclo ses an integrated screening system for sizing Wood chips.The system produces chips Within the acceptable size range, chips Which are below theacceptable size range (pin chips and fines), and over-thick chips, Which are subsequently typically transmitted to a chip size reduction apparatus such as a slicer.

Examples of prior art technology relating to methods and equipment for treatment ofWood room effluents are disclosed in US 4,535,284 (Saalasti), US 4,58l,l36 (Saalasti),and US 4,673,0l3 (Saalasti).

SUMMARY OF THE INVENTION The main object of the present invention is to provide a method of producing a sludgemixture that optimizes the utilization of on one hand pin chips, shives, f1nes and otherundersized f1brous material found in an undersized fraction from a Wood chip classif1erand on the other hand bio sludge emanating from the Wood room and/or pulp mill orpaper mill.

This object is achieved in accordance With the present invention as defined in theappended claims.

Thanks to the invention the sludge mixture Will comprise an enhanced amount ofcellulose f1bers, Which in tum enhances the deWatering of the biosludge so that a higherdry solids content of the biosludge is obtained.

According to one aspect of the invention, the f1brous mixture is thickened, therebyforming a thickened fiber sludge, before being mixed With the biosludge. Thanks to this aspect a lesser amount of Water has to be handled.

According to another aspect of the invention, the comminuted f1brous material issuspended in a Waste Water flow from the Wood room thereby taking care of solid matter and sludge comprised in the Wood room Waste Water.

According to still another aspect of the invention, the biosludge is thickened beforemixing it With the f1brous mixture or the thickened fiber sludge. Thanks to this aspect a lesser amount of Water has to be handled.

According to yet another aspect of the inVention, the sludge mixture is passed through adewatering equipment where said sludge mixture is dewatered before being bumt in thebumer. Thanks to this aspect the therrnal Value of the sludge mixture is substantially raised leading to a better heat efficiency of the bumer.

BRIEF DESCRIPTION OF THE DRAWINGSIn the following, the inVention will be described in more detail with reference to preferred embodiments and the appended drawing.

Figure 1 is a program flow chart illustrating the steps of an embodiment of the method according to the present inVention, Figure 2 shows a preferred embodiment of the inVention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In Fig. 1, woodchips 1 are supplied from a chipper, not shown, to a chip screen 2 forsorting into three fractions. A first fraction 3 is oVersized chips that are passed on to aslicer (not shown) for reduction to chips of a desired size. A second fraction 4 is chipsof the desired size, and this fraction usually is forwarded to a chip stack (not shown),from which the chips are forwarded to a digester (not shown) for cooking so as todissolVe lignin and free the f1bers. A third fraction 5 is an undersized fraction thatcomprises pin chips, shives, f1nes and other undersized f1brous material. This third,undersized fraction 5 is supplied to a size reducing mill 6, where the undersized materialis comminuted. The resulting comminuted f1brous material is then forwarded to a tankor basin 7, where the fibrous material is suspended in an, to the tank or basin 7,incoming waste water flow 8, said waste water flow preferably coming from the woodroom, thereby achieving an aqueous f1brous mixture 11. Said fibrous mixture isconveyed to a mixing tank 12 for mixing with a biosludge 13 thereby forrning a sludge mixture 14.

If desired, the biosludge may be thickened before the mixing with the f1brous mixture.The thickening may be carried out in a pre-sedimentation basin 15, from which water isrecirculated through a conduit 16 back to somewhere in the process, e.g. the wood room, while thickened biosludge 13 is conveyed to the mixing tank 12.

Thereafter, the sludge mixture 14 suitably is then led to a dewatering equipment 17 forpressing water out of the sludge mixture 14. The dewatering equipment preferablycomprises a screw press 17. A screw press is a machine having a conical screw workingin a conical housing with perforated screen plates for dewatering a suspension fed intothe conical space between the screw and the housing to get a product of high dry solidscontent. The water 18 is discharged from the screw press 17 and is recirculated to anappropriate location within the process. The compressed sludge mixture 14 now has adry solids content of preferably at least 20 wt%, more preferred 28 wt% and mostpreferred higher than 28 wt%. The higher content of fibers in the fiber sludge or fibrousmixture which is to be mixed with the biosludge, the higher dry solids content of thecompressed sludge mixture can be reached. A preferred mixture proportion betweenfibrous mixture or thickened fiber sludge and biosludge may be 0,8-1,0. The dewatered, compressed sludge mixture is then supplied to a bark boiler 19 for buming.

In Fig. 2 a preferred embodiment of the innovative method is shown. In the preferredembodiment the method comprises the same steps as already described in relation toFig. 1 but in addition also a step of thickening of the aqueous fibrous mixture. Saidthickening step is preferably performed after the step in which the comminuted fibrousmaterial is suspended in the incoming waste water flow 8 in the tank 7. The thickeningstep comprises conducting the aqueous fibrous material to a pre-sedimentation basin 9in which basin 9 the solids comprising the fibrous material, i.e. the cellulose fibers, inthe mixture are caused to settle. It is conceivable to pass one or several waste waterflows from different locations within the mill to said pre-sedimentation basin 9 so as totaking care of also cellulo se fibers that may be comprised in said waste water flow/ -s bythe same settling procedure. The solids-free water 10 is removed from the pre-sedimentation basin 9 and may be recirculated in the mill, preferably back to the woodroom but other locations may be conceivable as well. The settled solids are removedfrom the basin 9 and recovered as thickened fiber sludge 11 and conveyed to the mixingtank or basin 12 for mixing the thickened fiber sludge with biosludge 13 therebyforming the sludge mixture 14. Said sludge mixture 14 is thereafter treated as is alreadydescribed in relation to Fig. 1, i.e. led to the dewatering equipment 17 for dewatering the sludge, preferably by pressing water out of the sludge, and further on.

In said preferred embodiment, the aqueous fibrous mixture is concentrated/thickenedwith regard to the comprised fibers in said mixture. Preferably a pre-sedimentationbasin 9 is used for the purpose of thickening the fibrous mixture. In the pre- sedimentation basin 9 the solids, i.e. the comminuted undersized fibrous material, comprised in the fibrous mixture settle, and, when the fibrous material has settled, thesettled fibrous material is pumped away from the pre-sedimentation basin 9 as fiber sludge 11.

In a third embodiment (not shown) said fiber sludge resulting from the pre-sedimentation may be further thickened by further processing, preferably in one orseveral sedimentation basins arranged in connection to and preferably after the step ofpre-sedimentation in said pre-sedimentation basin 9. The fiber sludge may due to thisadditional sedimentation step reach an even higher dry so lids content. The dry solidscontent of the thickened fiber sludge may be about 3 wt%, but embodiments with higher or lower dry solids content of the thickened fiber sludge may be conceivable.

If desired, the biosludge may be thickened before the mixing with the thickened fibersludge. The thickening can be carried out in one or several sedimentation basin/ -s 15,from which water 16 is recirculated. Thickened biosludge 13 is conveyed to the mixingtank or basin 12. Typically, dry solids content of the thickened biosludge 13 may beabout 2,3 wt% but the thickened bio sludge may have higher as well as lower dry solids content.

All or substantially all waste water outlets from the pulp mill are taken care of byleading them to a waste water treatment plant which plant typically comprises one orseveral aerated lagoon/-s. In the waste water treatment plant microbes consume organicsubstances comprised in the waste water by total oxidation while simultaneouslyproducing biosludge. It is necessary to continuously remove an excess of biosludgefrom the waste water treatment plant in order to keep the concentration of the microbesat a certain level. The removed biosludge has to be taken care of. Usually, the biosludgeis bumt in a boiler, but since it may be hard or difficult to dewater the biosludge, itstherrnal value is low. However, by mixing the bio sludge 13 with the fibrous mixture 11,or, according to the preferred embodiment, with the thickened fiber sludge 11, thesludge mixture becomes easier to dewater. An increased dry solids content is achieved,probably because the fibers reinforce the sludge, so that more water can be pressed outof the sludge without simultaneously lo sing said biosludge, and the result is asubstantial increase of the therrnal value of the dewatered sludge. Furthermore, thanksto the increased dry solids content of the sludge mixture a larger amount of biosludgeper 24 hours counted as ton dry solids of biosludge per 24 hours, can be handled at the mill by the innovative method.

As more Water can be pressed out, the higher dry solids content achieved is favorable inthe subsequent burning in the bark boiler 19, which may preferably be a fluidized bed.A drier fiael (biosludge) will not forrn sticky deposits in the boiler 19 as easily as a fuelof lower dry solids content. In the bark boiler 19, bark, sludge, etc. is bumt. Also thetransport is facilitated, as a drier fuel will not adhere to the conveyers, and additionallyit is more economical to transport as little water as possible. Naturally, also the fueleconomy will be improved with a fuel having higher dry solids content. The chip fibersthat earlier were supplied directly to the bark boiler 19 for burning, now are exposed tosuspension in a waste water flow 8 coming from the wood room, settling in the pre-sedimentation basin/-s 9, mixing in a mixing tank 12 with biosludge 13, and dewateringin a dewatering equipment 17 before the f1bers finally are bumt in the boiler 19.Thereby, no f1bers are lost, but the f1bers only get a longer circulation path as aconsequence of their new task of being reinforcing material in the sludge mixture 14during pressing in the dewatering equipment 17, before the fibers are conveyed to the bark boiler 19 for buming.

The wood in the pin chips, shives, fines and other undersize material is preferablyhardwood. Hardwood generally has higher dry so lids content than softwood and,consequently, reduces tendencies of plugging in the hammer mill. A drier material willnot adhere as easily on the surface that the material comes into contact with. However, softwood may also be utilized by the innovative method The comminuting of the undersized material is preferably carried out in a hammer mill6. A hammer mill may be better suited than other types of size reducing mills for sizereduction of the undersized fraction from a wood chip classifier 2. Essentially, ahammer mill 6 comprises a steel drum (not shown) containing a vertical or horizontalrotating shaft or drum (not shown) on which hammers (not shown) are mounted. Thehammers are free to swing on the ends of the cross, or fixed to the central rotor. Therotor is spun at a high speed inside the drum while material is fed into a feed hopper(not shown). The material is impacted by the hammer bars and is thereby shredded and expelled through screens (not shown) of a selected size in the drum.

Suitably, the hammer mill 6 has a screen member (not shown) with openings of a sizethat is sufficient for restraining the comminuted f1brous material until a predeterrninedsize of the comminuted f1brous material is obtained. Thereby, the risk of splinters andshives plugging the screen plates (not shown) of any auxiliary drainers before the screw presses 17 will be reduced.

It is conceivable that in other embodiments of the invention it may be preferred to mixthe biosludge With the f1brous material and thereafter pass the mixture of biosludge andf1brous material forward to installations, e. g. sedimentation basins, for furtherthickening of the mixture before the final dewatering of the mixture to the compressed sludge mixture in some sort of dewatering equipment.

The invention also includes a device for performing the innovative method.

As Will be understood by those skilled in the present field of art, numerous changes andmodifications may be made to the above Without departing from the scope of thepresent invention as defined in the appending claims. For example, other comminutingequipment than a hammer mill may be utilized for the comminuting of the undersizedmaterial as Well as utilizing other deWatering equipment or pressing equipment than ascrew press for pressing Water out of the sludge, e. g. centrifuges, While still achievingthe benefits of the innovative method. Said pre-sedimentation step may comprise morethan one pre-sedimentation basin. It is further understood that the also other kinds of bark boiler may be used.

Claims (9)

1. . Method of producing a s1udge mixture (14) in a pu1p mi11, c h ar a ct e r i z e d in that said method comprises the steps of: a) supp1ying a size reducing mi11 (6) With an undersized fibrous material (5)comprising pin chips, shives, fines and other undersized material from a Woodchip c1assifier (2); b) comminuting said undersized material (5) in said size reducing mi11 (6); c) suspending the resulting comminuted fibrous material in a Waste Water flow (8)thereby achieving a fibrous mixture (11); and d) mixing said fibrous mixture (11) With a bios1udge to forrn a s1udge mixture (14). .

2. Method according to c1aim 1, c h a r a c t e r i z e d in that said method further comprises, before step d), the steps ofe) conducting said fibrous mixture to a pre-sedimentation basin (9),f) causing the so1ids comprised in the fibrous mixture to sett1e, and g) recovering the sett1ed so 1ids in said fibrous mixture as thickened fiber s1udge (ii).

3. Method according to c1aim 2, c h a r a c t e r i z e d in that said method further comprises, afier step g), the step of further thickening said thickened fiber s1udge(11) before step d).

4. Method according to any one of c1aims 1-3, c h ar a c t e r i z e d in that said Waste Water flow (8) is Waste Water flow (8) from a Wood room. .

5. Method according to any one of c1aims 1-4, c h ar a c t e r i z e d in that the bio s1udge (13) is thickened before mixing it With the fibrous mixture or the thickenedfiber s1udge. .

6. Method according to any one of the preceding c1aims, characterized in that step d) is followed by passing the s1udge mixture (14) through a deWatering equipment so asto deWater the s1udge and thereafter buming said deWatered s1udge mixture in abumer (19).

7. Method according to c1aim 6, c h a r a c t e r i z e d in that said deWatering equipment comprises a screw press (17).

8. 10. 11. Method according to any one of the preceding c1aims, c h a r a c t e r i z e d in thatthe Wood in the pin chips, shiVes, f1nes and other undersize materia1 is hardWood or softwood. Method according to any one of the preceding c1aims, c h a r a c t e r i z e d in that the comminuting is carried out in a hammer mi11 (6). Method according to c1aim 9, c h ar a c t e r i z e d in that the hammer mi11 (6) has ascreen member With openings of a size that is suff1cient for restraining thecomminuted fibrous materia1 until a predeterrnined size of the comminuted fibrous materia1 is obtained. DeVice for producing a s1udge mixture (14) in a pu1p mi11, c h ar a c t e ri z e d inthat said device comprises: a size reducing mi11 (6) arranged to comminute an undersized fibrousmateria1 (5) comprising pin chips, shiVes, f1nes and other undersized materia1 from aWood chip c1assif1er (2); a suspension tank (7) arranged to suspend the resu1ting comminuted fibrousmateria1 in a Waste Water flow (8) thereby achieving a fibrous mixture; and a mixing tank (12) arranged to mix said fibrous mixture With a bio s1udge to form a s1udge mixture (14).