METHOD AND APPARATUS FOR RECOVERING FIBRES FROM WHITE WATER OF PAPER MILL

Abstract: A method and an apparatus for recovering fibres from white water of a paper mill. In the method, filtering is carried out by conveying the white water through auxiliary pulp produced from a long-fibred fraction of the fibre material removed from a filter (2). The apparatus comprises a screen to fractionate the fibre material removed from the filter (2) into a short-fibred and a long-fibred fractions, and means for returning the long-fibred fraction to the filtering surface of the filter (2).

Description: METHOD AND APPARATUS FOR RECOVERING FIBRES FROM WHITE WATER OF PAPER MILL The present invention relates to a method for recovering fibres from white water of a papermaking process, the white water being separated from pulp mixture used for papermaking at different stages of the process, in which method the white water containing fibre material is, together with separate auxiliary pulp, conveyed to a filter such that the auxiliary pulp forms a layer on the surface of the filter and the white water flows through the layer while the fibre material remains in the layer formed by the auxiliary pulp, and in which method a mixture of the fibre material and the auxiliary pulp is removed from the filtering surface and the fibre material separated from the white water is returned to the process. The present invention further relates to an apparatus for recovering fibres from white water of a papermaking process, the apparatus comprising a filter equipped with a filtering surface, means for feeding the white water to the filter, means for feeding separate auxiliary pulp to the filter in order to form a pulp layer on the filtering surface, means for removing the auxiliary pulp formed on the filtering surface and fibre material separated from the white water from the filtering surface, means for conveying the filtered water from the filter, and a screen for fractionating the auxiliary pulp removed from the filtering surface and the fibre material separated from the white water into two fractions having different roughness. In the manufacturing process of a paper web or a similar fibre web, water is removed from pulp mixture at different stages so as to achieve appro- priate pulp and, eventually, a dry web. The water is at least partly recycled to the process, but it is possible that some of it must be removed to a waste water unit. Such filtrate water is separated through different slotted or perforated screens and wires at different stages of the papermaking process, and the filtrate water also takes useful fibre material therewith, the fibre material typi- cally being fine compared to the typical fibre structure of the pulp mixture. Fibre material is ordinarily recovered from such filtrate water by different filters wherein the filtrate water is conveyed through different filtering surfaces, such as cloth or the like, such that the fibre material remains on the surface of the cloth on the input side of the water and the water is removed from the other side of the cloth. Such devices include e.g. disc filters with large discs formed from filtering elements arranged next to each other. These filter- ing elements comprise a frame attached on the disc filter on the axle, the element being covered by a filter bag wherethrough the water flows into the axle through an opening located on the inside of the element at the end facing the axle. Drum filters, in which a wire operating as the filtering surface is provided on a cylindrical frame, are also used for the purpose. Such drum and disc filters and the structure and operation thereof are generally known per se to one skilled in the art; therefore, they will not be described in closed detail here. A problem with these solutions is that they should allow a large amount of water to pass through while adsorbing, however, fine fibres on the filtering surface. A solution which has been found efficient in practice is a method wherein a thin layer of pulp having longer fibres, so-called auxiliary pulp, is first built up on the filtering surface to form, as it were, a thicker filtering layer on the filtering surface on which the fine fibre then remains as the water passes through the layer formed by the auxiliary pulp. The problem with the use of the auxiliary pulp in the known solutions is the feeding of the auxiliary pulp to the device since in the beginning some of the fine fibre material passes through the filtering surface before the auxiliary pulp layer has been formed. Furthermore, the structure and composition of the auxiliary pulp greatly affect its functionality and the operation of the filter. Consequently, the fibre length distribution of the auxiliary pulp greatly affects its operation as the filtering layer. FI Patent 88732 discloses a solution wherein the auxiliary pulp to be fed to a filter is fractionated into a short fibre material part and a long fibre material part, of which only the part with the long fibres is fed as the auxiliary pulp into the filtrate water. This solution is quite difficult; furthermore, the fibre structure of the auxiliary pulp has to be such that the auxiliary pulp, as a mixture of the recovered pulp and the fine fibres, can be further fed to be used in the papermaking process. An object of the present invention is to provide a method and an apparatus to avoid difficulties of the known solutions, and extremely efficient and functional filtering before fibres are recovered. A method of the invention is characterized in that the mixture of the auxiliary pulp removed from the filter and the fibre separated from the white water is fractionated into a short-fibred and a long-fibred fractions, the short- fibred pulp fraction being returned to the papermaking process and the long- fibred pulp fraction being returned as the auxiliary pulp to be fed to the filter. An apparatus of the invention is characterized in that the screen is arranged to fractionate the mixture removed from the filter such that the fibre length of the pulp fraction having the shorter fibres is at most as great as the fibre length of the pulp mixture used in the papermaking process, whereby the short-fibred pulp fraction can be returned to the papermaking process, and in that the apparatus comprises means for returning the long-fibred pulp fraction to the filter in order to form a pulp layer on the filtering surface. The idea underlying the invention is that auxiliary pulp specially manufactured for the purpose is fed to a filter while the length distribution of the auxiliary pulp can be made to perfectly suit the filtering purpose regardless of what kind of pulp is used for the actual papermaking process. A further idea underlying the invention is that the auxiliary pulp is recycled in the filter such that a mixture separated from the filter of the auxiliary pulp and the fine fraction is fractionated into two parts, of which parts the one with the longer fibres is returned to be fed as the auxiliary pulp to the filter and the fraction with the shorter fibres is conveyed back to the papermaking process to be used therein. An advantage of the invention is that it allows a fibre length to be used which is advantageous to the filtering, in which case the fibre length can greatly differ from the one used in the actual papermaking process. Furthermore, addition of a relatively small amount of the auxiliary pulp suffices since the auxiliary pulp is recycled to the filter after fractionation carried out at the outlet end, and only the part of the fibres that is to be fed into the papermaking process is then forwarded. Furthermore, the use of the auxiliary pulp does not affect the actual main process since the auxiliary pulp can, if necessary, be manufactured completely separately, which means that the main process is allowed to operate continuously without interference. A further advantage is that when the fibres used as the auxiliary pulp have been worked enough, they are, with the short-fibred pulp fraction separated in the fractionation, trans- ferred to the papermaking process without causing any disturbance to the process. A still further advantage of the invention is that only a similar amount of the auxiliary pulp needs to be fed to the filter to the amount that is removed with the fines during fractionation, whereby on the whole, the necessary amount of the auxiliary pulp is quite small. The invention will be described in closer detail in the accompanying drawings, in which Figure 1 is a schematic block diagram showing an implementation of a method of the invention, Figure 2 is a schematic view of an apparatus suitable for implementing the method of the invention, Figure 3 is a schematic view of another apparatus suitable for implementing the method of the invention, and Figure 4 is a schematic block diagram showing another implementation of the method of the invention. Figure 1 is a schematic block diagram showing a method of the in- vention. In the method, the filtrate water is fed via an inlet channel 1 to a filter 2. Auxiliary pulp, which forms layers on the filtering surface of the filter 2, collecting fibre brought along with the filtrate water, and which is removed therefrom with the short-fibred friction in a normal manner, is also fed to the filter 2 via a bypass channel 3. The water separated from the filter 2 is removed via an outlet channel 4 to a suitable place either to be returned to the papermaking process or to a waste water treatment plant. The pulp separated from the filtering surface of the filter 2 is conveyed to fractionation 5 to be screened into a short-fibred and a long-fibred fractions. The short-fibred fraction is forwarded via a supply channel 6 to be fed back to the papermaking process, and the long-fibred fraction is returned via the bypass channel 3 to the filter 2. In fractionation, the fractionation of the pulp into long and short fibres is dimensioned such that the short-fibred pulp fraction is at most equal in length to the longest fibre fraction of the pulp used in the papermaking process. Hence, it can be fed directly into the ordinary pulp with no effects on the characteristics of the paper being produced or a similar web. New auxiliary pulp is, if necessary, fed to the filter 2 via an auxiliary-pulp channel 7 continuously at a steady pace or at suitable intervals as the amount of the auxiliary pulp used in the filter is reduced as the fibres thereof are worn shorter, and, eventually taken to the fine fraction in fractionation. Figure 2 schematically shows an apparatus for implementing the method of the invention. The Figure discloses a filter 2, which is a filter known per se, such as a disc filter or a drum filter, the filtrate water with its fibres being fed to a basin 8 around the filter. The auxiliary pulp necessary for the operation can be fed to the basin with clean water before any other functions are initiated, so that the filtrate water and the small fibres therein cannot pass the filter before an auxiliary pulp layer has been formed on the surface of the filter. If necessary, the water separated by the filter can be recycled to the filter via an activation channel 9, in which case by recycling both the auxliliary pulp and the water the filter can be activated before it is necessary to convey the filtrate water separated in the papermaking process to the filter. When the filtrate water is fed to the filter, it is conveyed to the basin 8 wherein the level of the surface is retained such that the water passed through the filtering surface flows into a channel formed in the axle of the filter and via the channel away from the filter. This "clean" filtrate water can be forwarded either to be returned to the pulp manufacturing process or, if neces- sary, to be discharged. From the filtering surface, the auxiliary pulp and the fine fibre brought along with the filtrate water are removed in a manner known per se and conveyed as a pulp suspension to fractionation wherein it is screened into said short and long fibre fractions by using a known screen per se, for example. The long-fibred fraction is returned via the recycling channel 3 to the basin 8 and the short-fibred fraction is forwarded via the supply channel 6. Figure 3 schematically shows another apparatus suitable for implementing the method of the invention. In this embodiment, a gap filter 10, which has a structure of a gap converging in the direction of flow, is used as the filter. In the direction of flow of the material to be screened, filtering wires 12 move at both sides of a gap 11 , whereby water is removed through the wires 12 and supporting surfaces 13 located outside the wires and equipped with apertures or openings. In order for the auxiliary pulp to form layers on the surface of the wires 12, the input of the gap filter is divided into three input channels 14a, 14b and 14c such that the auxiliary pulp from the bypass channel 3 is fed in via the outmost input channels 14a and 14c facing the wires, and only after an auxiliary pulp layer has been formed, is filtrate water 15 from the middle channel 14b conveyed between auxiliary pulp layers 14, whereby the water is allowed to flow away only through the auxiliary pulp layers 14. This structure enables extremely efficient screening since a desired auxiliary pulp layer is produced on the surfaces of the wires 12 already before the filtrate water to be screened is allowed to discharge on the surfaces. Simultaneously, a desired thickness of the auxiliary layers 14 and exact desired filtering characteristics can be achieved. Consequently, the screening provided by the gap filter is extremely accurate, the capacity at the same time being extremely high. Figure 4 is a schematic block diagram showing a method of the invention. In the figure, numbers 10 and 16 schematically show a gap filter and a gap screen, respectively, whose structure and operation are in principle of the type shown in Figure 3. The only essential difference is that no separate auxiliary pulp is fed to the gap screen on the surface of wires but the pulp to be screened is fed as such between wires 12b of the gap screen 16. In the method, the pulp separated from the gap filter from the surface of wires 12a is conveyed to fractionation where it is fractionated into a long-fibred and a short- fibred fractions, i.e. into an auxiliary pulp fraction 18 and a fraction 17 to be conveyed to the papermaking process. The gap screen 16, for example, whereto the pulp suspension to be fractionated is fed can be used for the fractionation without forming any auxiliary layer on the surface of its wires 12b. This embodiment enables the short fibres to be returned to the papermaking process and some water to be removed through the wires 12b, whereas the long fibres remain on the surface of the wires 12b wherefrom they can be returned to the gap screen 10 via the bypass channel 3. Such a unity comprising the gap filter and the gap screen is extremely efficient both in terms of capacity and screening accuracy and it takes extremely little space compared to large screening drums and disc screens. Furthermore, the operating costs of such devices are quite low since ordinary wires can be used instead of expensive special wires or filter bags. Also in this embodiment utilizing the gap filter, operation can be started by means of the auxiliary pulp only by recycling the water and the auxiliary pulp until the filtrate water to be screened is fed to the gap filter. For fractionation of the detached long and short pulp mixture re- moved from the gap filter, other ordinary screens, such as cylinder screens or the like, that are commonly known per se can naturally also be used. In practice, however, the unity comprising the gap filter and the gap screen is clearly more efficient, both technically and economically. The invention has been described above in the description and the drawings only by way of example and it is by no means restricted thereto. The essential point of the invention is that for separating the fines from filtrate water, separate long-fibred auxiliary pulp is used in a filter, whereafter a mixture of the auxiliary pulp and the fines is fractionated into the long-fibred auxiliary pulp and a fraction having shorter fibres suitable to be fed to the pulp process, the long-fibred auxiliary fraction being returned to a screen in order to form a filtering layer on the surface of the filter.

Claims: CLAIMS 1. A method for recovering fibres from white water of a papermaking process, the white water being separated from pulp mixture used for papermaking at different stages of the process, in which method the white water containing fibre material is, together with separate auxiliary pulp, conveyed to a filter (2) such that the auxiliary pulp forms a layer on the surface of the filter and the white water flows through the layer while the fibre material remains in the layer formed by the auxiliary pulp, and in which method a mixture of the fibre material and the auxiliary pulp is removed from the filtering surface and the fibre material separated from the white water is returned to the process, characterized in that the mixture removed from the filter (2) of the auxiliary pulp and the fibre separated from the white water is fractionated into a short-fibred and a long-fibred fractions, the short-fibred pulp fraction being returned to the papermaking process and the long-fibred pulp fraction being re- turned as the auxiliary pulp to be fed to the filter. 2. A method as claimed in claim 1, characterized in that the auxiliary pulp is produced from separate pulp having substantially different characteristics from the pulp mixture of the papermaking process. 3. A method as claimed in claim 1 or 2, characterized in that the pulp removed from the filter (2) is fractionated such that the fibre length of the short-fibred pulp fraction is at most the greatest fibre length of the pulp used for the papermaking process. 4. A method as claimed in any one of the preceding claims, characterized in that the fibre length of the auxiliary pulp is at least two times the fibre length of the pulp used for the papermaking process. 5. Λ method as claimed in any one of claims 1 to 3, c h a r a c - t e r i z e d in that the auxiliary pulp is fed to the filter to form the layer on its filtering surface before the white water is fed to the filter. 6. A method as claimed in any one of the preceding claims, characterized in that a disc filter or a drum filter is used as the filter. 7. A method as claimed in any one of claims 1 to 5, characterized in that as the filter is used a gap filter (10) comprising a gap (11 ) which converges in the direction of propagation of the white water to be filtered, and, at both sides of the gap, wires (12) which move in the same direc- tion, forming the filtering surfaces of the filter. 8. A method as claimed in claim 7, characterized in that the auxiliary pulp is fed on the surface of the wires (12) facing the gap (11) via separate input channels (14a, 14c) and the white water to be filtered is fed therebetween, whereby the auxiliary pulp forms a pulp layer (14) on the sur- face of the wires (12) before the white water is allowed to come into contact with the auxiliary pulp. 9. A method as claimed in claim 7, characterized in that the auxiliary pulp and the white water to be filtered are mixed before they are fed to the surface of the wires (12) facing the gap (11 ). 10. An apparatus for recovering fibres from white water of a paper- making process, the apparatus comprising a filter (2) equipped with a filtering surface, means for feeding the white water to the filter, means for feeding separate auxiliary pulp to the filter in order to form a pulp layer on the filtering surface, means for removing the auxiliary pulp formed on the filtering surface and fibre material separated from the white water from the filtering surface, means for conveying the filtered water from the filter, and a screen (5) for fractionating the auxiliary pulp removed from the filtering surface and the fibre material separated from the white water into two fractions having different roughness, characterized in that the screen (5) is arranged to frac- tionate the mixture removed from the filter (2) such that the fibre length of the pulp fraction having the shorter fibres is at most as great as the fibre length of the pulp mixture used in the papermaking process, whereby the short-fibred pulp fraction can be returned to the papermaking process, and in that the apparatus comprises means for returning the long-fibred pulp fraction to the filter (2) in order to form a pulp layer (14) on the filtering surface. 11. An apparatus as claimed in claim 10, characterized in that the auxiliary pulp used in the filter has substantially different characteristics from the pulp mixture used in the papermaking process. 12. An apparatus as claimed in claim 10 or 11, character- i z e d in that the filter is a disc filter or a drum filter. 13. An apparatus as claimed in claim 10 or 11, characterized in that the filter is a gap filter (10) comprising a gap (11) which converges in the direction of propagation of the white water to be filtered, and, at both sides of the gap, wires (12) which move in the direction of flow of the white water, the auxiliary pulp forming the pulp layer (14) on the surfaces of the filtering wires and the white water to be filtered being conveyed to flow therethrough. 14. An apparatus as claimed in claim 13, characterized in that the gap filter (10) comprises three input channels (14a to 14c) between the wires (12) such that the auxiliary pulp is fed to the surface of the wires from the outermost input channels (14a, 14c) and the white water is fed from the channel (14b) therebetween such that the auxiliary pulp has already formed the pulp layer (14) on the wires before the white water is allowed to come into contact with the auxiliary pulp. 15. An apparatus as claimed in any one of claims 10 to 13, characterized in that the auxiliary pulp and the white water to be filtered have been mixed before they are fed to the filter. 16. An apparatus as claimed in any one of claims 10 to 15, characterized in that the screen (5) is a gap screen (16) whereto the mixture of the auxiliary pulp and the pulp separated from the white water is fed.

Inventors: JUUTINEN VESA, PEKKARINEN TIMO, JUUTINEN, VESA, PEKKARINEN, TIMO
Applicants: JUUTINEN VESA, PEKKARINEN TIMO, VALMET CORP, JUUTINEN, VESA, PEKKARINEN, TIMO, VALMET CORPORATION
Application year: 2000
Main classification: D21F1
IPC codes: B01D33, B01D36, B01D37, D21F1, D21F9