WO1994017242A1

WO1994017242A1 - Process and apparatus for separating fibres from water

Info

Publication number: WO1994017242A1
Application number: PCT/FI1994/000021
Authority: WO
Inventors: Paul Olof Meinander
Original assignee: POM Technology Oy AB
Current assignee: POM Technology Oy AB
Priority date: 1993-01-21
Filing date: 1994-01-20
Publication date: 1994-08-04
Anticipated expiration: 1995-07-21
Also published as: JPH08509526A; FI91980B; FI930247A0; FI91980C; AU5861394A; EP0681626A1

Abstract

The separation of fibres from a fibre-containing water is performed by means of continuous flotation, in an apparatus (10) comprising a mantle (15) of circular cross section and means (12) for causing said fibre-containing water to rotate in a vortex on the inner surface of said mantle (15). Finely distributed gas is fed through a gas permeable part (18) of said mantle (15) into said vortical water. A foam column is generated in the center of said mantle (15) and is discharged through a discharge pipe (13). Said discharging pipe (13) is preferably connected to a foam extinguishing apparatus (30). The process and apparatus are preferably used for the recovery of fibres from backwater in a papermachine.

Description

PROCESS AND APPARATUS FOR SEPARATING FIBRES FROM WATER

The present invention relates to a process for separating fibres from fibre-containing water, particularly from back¬ water circulating in the short circuit of a papermaking machine. The invention also relates to an apparatus for separating fibres from fibre-containing water by means of flotation and to the use of said apparatus for the fast separation of fibres from backwater.

The papermaking process is traditionally designed for achieving the best possible stability, so that the influence of possible process disturbances has time to balance. At the same time, however, the controllability of the process is slow, and especially at grade changes stabilizing the new grade takes several minutes. Changing the color of the paper may last even more than an hour.

In commonly known papermaking processes the paper or paper- board web or the like is formed by filtering excessive water from stock through a wire cloth, and recycling the main part of the filtered water into the so called short circulation. The stock flow is fed to the backwater pit of the short circu¬ lation and further through cleaning devices, like centrifugal cleaners and screens to the head-box. The recyclings and feed¬ backs, frequently made in multiple stages, make the system complicated and slow. In addition, the huge and partly un¬ defined water volumes and slow flow speeds needed for avoiding the intrusion of air into the water, emphasize the slowness of the process and make it difficult to manage accurately.

A papermachine wet end produces an excess of water, which is normally brought to previous stages of the process as a so called long circuit. The fibres and other solids contained in said water are normally separated from the water in order to get them back into the proper fibre process as soon as possible. Generally filters, sedimentation basins or flotation devices are used as means for separation or recovery of the fibres.

In a filter, backwater is filtered through a porous layer of pulp, and solids contained therein are returned into the process together with the filtering pulp. In sedimentation basins, on the other hand, the solids slowly settle at the bottom of the basin, from where they are conducted back to the process. In flotation devices, of which there is a great number of commercial types, the backwater normally is satu¬ rated with air, or pressurized air is fed into it. The air is distributed as small bubbles, onto which the solids adhere due to surface tension. The solids then float toward the surface of the water together with said bubbles. The foam containing fibres is then removed from the surface and the contained fibres are brought back into the process via foam extinguishing.

The weakness of all of these fibre separation methods is their slowness, or the relatively long time required between the separation of backwater and the return to the process of fibres contained therein. Further, they comprise complicated equipment having many corners exposed to deposit of dirt and requiring complicated stock handling. All of this makes the process sensitive to disturbances and difficult to manage.

During stable process conditions said slowness does not cause great harm, but if, at a change of the produced paper grade or for other reasons, one wants to change process parameters, said slowness renders the process more difficult to control. Moreover, the flotation and sedimentation processes are especially sensitive and exposed to disturbances. The recovery of fibres and solids is often unstable, which causes material losses as well as process disturbances and variations in the paper quality.

When aiming at a fast grade change, as proposed in co-pending patent application WO 93/23612 by the same inventor and in¬ cluded herein by reference, the traditional fibre recovery devices are a serious obstacle to the fast control of the process. It is typical that even if a fast grade change as such may succeed, that is, the produced paper will meet the specification, still after a time, ranging from half an hour to several hours, the return flow of recovered fines will change heavily. This causes also the draining and drying characteristics of the paper web to change, which may cause a web break or significant waste production at the paper machine.

Flotation and sedimentation are used also in other processes, among others for ore dressing in metallurgy. In US Patents 4,279,743; 4,397,741 and 4,399,027 Jan P. Miller has presented a process for dressing ore in a compact space in a gas sparged hydrocyclone. Finely distributed gas is pressed through a rotating suspension, which causes an accelerated flotation inside the cyclone. Ahlstom-Kamyr Inc. , Glenns Falls, NY, USA has used this method for removing printing ink from waste paper. Hereby the ink particles adhere to small air bubbles filtering through a fibre suspension of 1...3% consistency and are rinsed with said bubbles through the fibre suspension. On the other hand, fibres in a suspension of said consistency form a net-work, which tie the fibres to each others. Thus, the relative position of the fibres does not change due to the influence of the air bubbles. The small ink particles can, however, move relatively freely together with the air bubbles in such an immobilized fibre network, and thus be removed from the fibre material to be cleaned, whereas the cleaned fibres remain in the suspension.

In said processes the removal of ink and separation of minerals are based on the hydrofobicity of the particles to be separated.

In traditional flotation processes for separating fibres, a very diluted suspension having less than about 0, 2% fibres is used. In such a diluted suspension the fibres do not consti¬ tute an immobilized network, but can move freely towards the surface, conducted by the bubbles. A fibre-enriched foam forms at the surface due to the fact that the air, in a certain sense, displaces water. Said foam reaches a relative high consistency, generally of several percent. The critical con¬ sistency, under which the fibres are freely mobile, varies according to the fibre length, and also with the shearing forces acting on the suspension. At low shearing forces said consistency may vary between about 0, 2% and 0, 02%. The fibres draining through the wire of a paper machine are normally relatively short, wherefore the critical fibre consistency of papermachine back-water is normally about 0, 2%. In the following description and claims fibre-containing water is intended to include waters, having a fibre consistency sufficiently low for the fibres not to form networks immobi¬ lizing the fibres.

In traditional flotation the fibre-containing water is fed into a device which is open or otherwise has a slow flow speed. Air is dispersed into the water and adheres to the fibres slowly floating them toward the surface. Compact devices, like gas sparged hydrocyclones have, however, not been used for the separation of fibres. The fibres are not hydrophobic particles like ink or minerals. Further, it is known that hydrocyclones form a foam which contains a very huge amount air. The air contained in the foam causes actual problems in later phases of the process, and the fibres cannot be returned to the process without removing the air. Due to the state of development of other equipment for the traditio¬ nal paper industry, there also has not been a need for a very fast and compact fibre recovery, because the compactness of the entire process has not been an important objective.

Said earlier patent application WO 93/23612 by the same inventor discloses an invention according to which the wet end process of a papermachine is made faster. The utilization of said invention can be further improved by rending faster also the recycling of recovered fibres to the fibre process.

An object of the present invention is to provide an apparatus and a process for fast and efficient separation of fibres from fibre-containing water.

An object of the invention is also to provide an apparatus and a process for separating fibres from the back-water of a papermachine and for recovering the fibres for further use in the papermaking process.

An object of the invention is particularly to provide an apparatus and a process for the separation of fines from the back-water of a papermachine which process is faster and more efficient than the prior art processes, and for the fast returning of said fines to the production process.

An object of the invention is also to provide a papermaking process, which can be more swiftly regulated and more accurate¬ ly controlled than traditional papermaking processes.

An object of the invention is further to provide a papermaking process, with a backwater circulation system, which allows the escape of relatively minor amounts of fibres and fillers, or none at all.

The process according to the present invention is based on the finding, that air can enrich fibres from a diluted fibre- containing liquid in a compact hydrocyclone. By thereafter removing said air from the foam produced in the hydrocyclone, it is possible to immediately pump said fibres back into the process.

Thus, the present invention relates to a process and an apparatus for separating fibres from a liquid, as defined in the appended claims, and to the use of said apparatus in a papermaking process.

Particularly, the invention relates to a process for separa¬ ting fibres from fibre-containing water by means of a con¬ tinuous flotation process, wherein said fibre-containing water is caused to rotate in a vortex inside a mantle, shaped essentially as a body of revolution; finely distributed gas is fed into said vortical water; the generated fibre-enriched foam is caused to move toward the center of said vortex; and fibre-enriched foam is removed from the fibre enriched foam column generated in the center of said vortex. Thereafter said fibre-enriched foam is preferably subjected to foam extinguishing.

The present invention also relates to an apparatus for separating fibres, which apparatus comprises a essentially circular mantle and means for causing fibre-containing water to rotate in a vortex movement at the inner surface of said mantle. Said apparatus further comprises means for feeding gas into said water, which means comprise a gas distribution part of said mantle for feeding finely distributed gas into said water forming said vortex, and means for removing fibre- enriched foam from a foam column generated in the center of said mantle. The apparatus is preferably connected to a foam extinguishing device.

The mantle of the fibre separation device according to the present invention is preferably shaped as a body of revolution, although it may also have a shape slightly deviating from such a shape. Thus, said mantle is preferably shaped as a cylinder or a truncated cone, but in certain embodiments it may be shaped as a multi-cornered polygon or a star providing a slight mixing in the water. Said mantle may also comprise one or more helical grooves in its surface. It is important that the shape of the mantle does not hinder the formation of a vortex in said mantle and that the surface of said mantle allows an unrestricted and fast rotation of the water.

In a preferred embodiment of the invention, said mantle is rotatable, thus providing or maintaining the rotation of said water vortex. Said rotatable mantle may further be connected to a pumping wheel for increasing the pressure of the fibre- depleted water gathering at the surface of said mantle, and for pumping said water forwards.

The gas distribution part of the mantle is preferably composed of a porous gas permeable portion of the mantle, but the gas distribution may also be provided by perforations or slots, or even by a piping ending in fine nozzles on the surface of the mantle.

The invention, together with additional objects and advantages thereof, as well as alternative structures and processes will be best understood by those skilled in the art from the following description, when read in connection with the accompanying drawings, wherein:

Fig. 1 shows a schematic axial section of a fibre separation apparatus according to a favorable embodiment of the invention;

Fig. 2 shows a section of the fibre separation apparatus according to Fig. 1 along line A-A;

Fig. 3 shows a section of a foam extinguishing device con¬ nected to the fibre separation apparatus of Fig. 1 along line B-B;

Fig. 4 shows a section of the foam extinguishing device of Fig. 3 along line C-C; Fig. 5 a section of a fibre separation apparatus according another favorable embodiment, where the foam column and the cleaned fluid are removed at the same end of the cyclone;

Fig. 6 schematically shows a section of a further embodiment of the invention, where the cyclone has an active rotor;

Fig. 7 shows a section of fibre separation apparatus according to Fig. 6 along line A" -A" ;

Fig. 8 shows a fibre separation apparatus according to a preferred embodiment of the invention, with a rotating mantle;

Fig. 9 schematically shows a papermachine backwater circu¬ lating system, wherein the invention is applied.

In the various drawings, the same reference numerals refer to corresponding parts of different embodiments.

Fig. 1 generally represents a fibre separation apparatus or gas sparged hydrocyclone 10, which comprises an essentially cylindrical mantle 15 of the cyclone and a spiral inlet 12 at one end thereof. In the center of said cylindrical mantle 15 there is an outlet for fibre-enriched foam or a foam pipe 13. At the opposite end from inlet 12 there is a vortex stabilizer 26 and an outlet 28 for water. At the inlet end 12 of the apparatus, the mantle 15 and the foam pipe 13 define between themselves an annular inlet nozzle 14. At the outlet end the mantle 15 and the vortex stabilizer 26 define between them¬ selves an annular outlet slot 27, which is connected to outlet 28. Said mantle 15 comprises a gas permeable part 18 made of porous material, which is entirely surrounded by a gas chamber 19.

The part 18 of mantle 15 is made of a sturdy but simultaneous¬ ly porous material so that it is permeable to gas. Said gas is discharged at the inner surface of part 18 of mantle 15 in a very finely distributed form. The gas permeable part 18 forms a major part of the entire mantle 15 so as to achieve an efficient mixing in the cyclone of air into water.

Fig. 2 shows a cross-section of fibre separation apparatus 10, showing the porous part 18 of mantle 15 and the surrounding gas chamber 19. The inner surface of porous part 18 consti¬ tutes a gassing surface 16, where the finely distributed gas discharged through the porous part during operation of the apparatus, mixes into the water rotating on the surface, and generates an annular flotation zone 17.

Further, the embodiment of Fig. 1 and sectional drawings Fig. 3 and Fig. 4 show a foam extinguishing device 30 which is favorably connected to fibre separation apparatus 10 by said foam pipe 13. Said device corresponds to a gas separation pump with means for extinguishing foam according to co-pending patent application WO 93/23135 by the same inventor and in¬ cluded herein by reference. Another such device is presented in co-pending patent application Fl 935853 by the same inventor. It is, however, evident that any other suitable foam extinguishing device can be used.

The foam extinguishing apparatus 30 according to Fig. 1 essentially comprises a conically expanding housing 38 in¬ cluding a rotatable rotor 32. Said housing is preferably formed as a truncated cone or a funnel. Its cross-section is preferably circular, although it may have a shape which slightly deviates from a perfect circle. Thus, it may be slightly angular or flow-obstacles may be applied at its inner surface. Such obstacles may comprise ridges, bumps or other deviations from the form of a body of revolution. A pumping chamber 45 and an outlet 42 are provided at the wider end of housing 38. The rotor 32 consists of a rotor shaft 34, rotor vanes 36 and winglets 37.

Foam extinguishing nozzles 42, which are connected to an external source of fluid through channel 41 in rotor shaft 34, are preferably applied to rotor 32. Since the rotation of rotor 32 causes the sprays to follow an arcuate pattern relative to the rotor, the rotational speed is taken into account when directing the nozzles so that the foam extinguishing spray 43 is directed at the foam vortex rotating on the inner surface of the housing, and not at the rotor blade 36. The extinguishing nozzles 42 may alternatively be arranged so that they are brought closer to housing 38, by means of a separate pipe fixed to the rotor and rotating with the same. The nozzles 42 may also be attached to the rotor blades 36. Foam extinguishing sprays may further be arranged with nozzles at the outlet end of the foam extinguishing device as described in said co-pending patent applications WO 93/23135 and Fl 935853 by the same inventor.

The foam extinguishing device 30 preferably has an air outlet 46 connected to a vacuum source, which may be, for example, a vacuum pump provided with a condenser.

In the embodiment of Fig. 5 the inlet 12 is at the opposite end of cyclone 10 in relation to foam pipe 13. The centerpiece 29 at the same end of the cyclone as inlet 12 and the mantle 15 define between themselves a feed slot 14. In the embodiment of Fig. 5 the centerpiece 29 is of a tapering shape but it might^* as well have a different shape. The foam-pipe 13 is provided with thick walls, whereby a sufficient stabilizing surface is obtained, and the end of foam pipe 13 acts as a stabilizer for the foam column. The mantle 15 and the foam- pipe 13 further define an annular slot 27, which is connected to outlet 28.

Figures 6 and 7 show an embodiment, wherein a rotor 50 is mounted inside cyclone 10. Said rotor is rotatably journaled and sealed at the inlet 12 of said cyclone, and its shaft 52 is driven in a well known way. Blades 56 are favorably attached to the centerpiece 54 of rotor 50. Fig. 6 further shows an extension of foam-pipe 13, which constitutes a foam extinguishing device 60 provided with nozzles 62, fed by a pipe 64. A part of, or preferably all the nozzles are connected to outlet tube 28 via return pipe 66, the flow of which is controlled by means of a control valve 66 or other known means. The extension of foam-pipe 13 is preferably connected to a further foam extinguishing device (not shown).

Fig. 7 shows a section A" -A" of the fibre separation apparatus of Fig. 6, where the centerpiece 54 and blades 56 of rotor 50 are seen more clearly.

Fig 8 shows a preferred embodiment of the invention, where the mantle 15' is rotatable inside a gas chamber 19, having a gas inlet 22. Mantle 15' may be brought into rotation by a shaft 82 which is driven by any known means. A part 18 of mantle 15' is made of a porous material which is permeable to gas and finely distributes the gas into a rotating vortex forming an annular flotation zone 17 on gassing surface 16.

Close to inlet 12 mantle 15' is preferably provided with blades 85 or other means, preferably rotating together with mantle 15' for bringing a liquid entering through inlet 12 into rotation with mantle 15' so as to form a vortex in the cyclone. At the opposite end of the rotatable mantle 15' there is a pump rotor 87 and outside said rotor 87 a pump housing 88, with an outlet 28 for fibre-depleted water. The pump rotor 87 is preferably fixed to the mantle 15' for rotating with the same. In this embodiment the foam-pipe 13 is fixed to the rotor 87 and rotatable together with said rotor. In other embodiments, however, the foam-pipe may be stationary. The shaft 82 may extend through the entire mantle 15' and pump rotor 87, being journaled at an extension of foam-pipe 13, or it may have bearings at one end only. The process according to the present invention is described in the following, referring specially to the embodiment of Fig. 1. The fibre-containing water to be cleaned, which is preferably back-water drained through a papermachine wire, is fed into inlet 12 of gas sparged hydrocyclone 10. The liquid is brought into a strong vortical movement, due to the narrowing spiral shape of inlet 12, and flows as a vortex through annular nozzle 14 into the cyclone, where the centri¬ fugal forces cause it to be pressed as a vortex against mantle 15.

Gas is pressed or sucked into the water vortex through the porous part 18 of mantle 15, whereby finely distributed gas is mixed into the water at gassing surface 16. Due to the centri¬ fugal forces, the gas spreads into the vortical fibre- containing water forming a flotation zone 17, where finely distributed air bubbles adhere to the fibres and to any fillers present. The finely distributed air bubbles move toward the center of cyclone 10, where a foam column 20, en¬ riched with fibres and fillers is formed.

A pressure or a vacuum is favorably applied to foam column 20 causing it to pass through foam-pipe 13 to foam extinguishing apparatus 30, attached to the cyclone 10. The water, depleted of fibres by the action of the gas, passes along the mantle 15 through annular slot 27 to water outlet 28.

In the situation shown in Figs. 1 and 2, the flotation zone 17 extends all the way to the vortex stabilizer 26, so that the foam column 20 and the fibre-depleted water do not come into immediate contact with each other. At a direct border between foam and water, a mixing of fibres into the water and, con¬ sequently, a return of fibres into the fibre-depleted water, might occur. In order to avoid such fibre losses a situation where flotation zone 17 extends all the way to vortex stabilizer 26 is sought by adjusting the operating parameters accordingly. From fibre separation apparatus 10 the foam is brought into foam-pipe 13 and preferably further to foam extinguishing apparatus 30, where the rotating rotor 32 and the tangential arrangement of the inlet cause the foam to rotate rapidly against foam extinguisher mantle 38. The centrifugal forces and mechanical stress cause the foam to collapse, forming a fibre suspension ring against mantle 38.

Due to its relatively high fibre content, the fibre-containing foam is rather stable. The extinguishing of foam may be pro¬ moted by spraying water on the foam through foam extinguishing nozzles 42 attached to the rotor providing showers 43. The extinguishing of foam may also be promoted by other known means by subjecting the foam to mechanical stress. The dilution of the foam by water also causes the stability of the foam to decrease. The foam extinguishing water brought to nozzles 42 is preferably fibre-depleted water from fibre separator 10 or any other water, but also other fluids like air or another gas or liquid can be used. The gas extinguishing is particularly efficient if a foam extinguishing chemical, of which several commercial products are available on the market, is added.

For improving the deaeration, a vacuum is preferably applied to the foam extinguishing apparatus 30 by connecting its air outlet 46 to a vacuum source. Hereby also foam column 20 and thus also the cyclone 10 are subjected to vacuum, which pro¬ motes the suction of gas through the porous part 18 of mantle 15 into flotation zone 17. Thus, the pressure difference required can be obtained either by feeding pressurized gas through gas inlet 22 or correspondingly by sucking gas through outlet 46 or by combining these two methods.

In the embodiment of Fig. 5 the fibre-containing water is fed into feeding slot 14 through the inlet 12 at the bottom end of cyclone 10. In this embodiment, the tapering center piece 29 gives space for the entering gas. The main task of said center piece is to promote the forming of the vortex.

By altering the shape of the center piece, an acceleration of the water flow can be obtained which provides space for the gas. It is not even necessary that the flotation zone touches center piece 29 as shown in Fig. 5, and the center piece may even end immediately after the starting point of the vortex.

The flotation continues until the air fed into flotation zone 17 has reached foam column 20 due to the centrifugal forces. In Fig. 5 this takes place before the flowing flotation zone reaches foam-tube 13, whereby a boundary layer 23 is formed between foam column 20 and the fibre-depleted water. Foam pipe 13 also functions as a vortex stabilizer and is provided with thick walls for providing a sufficient stabilization surface.

At boundary layer 23 a certain mixing may occur, which may return fibres into the fibre-depleted water thus removing them as fibre losses from the process, or mix cleaned water into the foam column 20 to be recovered. As this decreases the efficiency of the apparatus, the operating parameters are preferably adjusted so as to provide a situation where the boundary layer 23 is as short as possible. The fibre-depleted water is discharged from the cyclone to outlet 28 through annular slot 27.

Even though' the preceding text, for the sake of simplicity, has referred to the top and the bottom end of the cyclone, it is obvious for persons skilled in the art that the function of the apparatus of the present invention does not depend on its position, but that it can be placed for example horizontally or at an angle. Further, it is obvious that it is irrelevant at which end the foam and clean water is removed and from which end the fibre-containing water is fed into the appara¬ tus. In the embodiment of Figs. 6 and 7, the rotating blades 56 of rotor 50 positioned inside cyclone 10 keep the fibre-contai¬ ning water, the flotation zone 17, the foam column 20 and the fibre-depleted water in a uniform rotational movement. In order to render foam extinguishing more efficient water, gas or another fluid is sprayed onto the passing foam through nozzles 62 in a separate foam extinguishing apparatus, formed as an extension of foam-pipe 13, said nozzles being fed by a pipe 64. The use of water is particularly favorable, because it will simultaneously dilute the foam, which is stable due to its high fibre content. As a foam extinguishing fluid it is preferable to use papermachine backwater which thus will return into circulation together with the recovered fibres. The foam which as been first extinguished by nozzles 62 is preferably brought to a second foam extinguishing apparatus, which may be of a similar type as previously described in connection with Fig. 1.

Especially in this embodiment, but also in other embodiments, the rotational energy of the fibre-depleted water can be re¬ covered as pumping energy by shaping the outlet 28 to form a pumping chamber, whereby the blades 56 of rotor 50 can be used as a pump rotor.

The fibre-containing water entering cyclone 10 is split into a fibre-enriched foam column 20 and fibre-depleted water in a proportion, which is mainly determined by the characteristics of the foam column 20. Said characteristics are determined by the amount of introduced air and the amount and characte¬ ristics of the separated fibre material. The amount of clear water leaving the fibre recovery unit, may be adjusted by returning a part of the water discharged through outlet 28 through return pipe 66 to foam-pipe 13, whereby the clear water returning from return pipe 66 is favorably used as one or more foam extinguishing showers 63. The amount of returning water is controlled by means of control valve 68 or by other known means. In the embodiment of Fig. 8 the rotation of fibre-containing water and the forming of flotation zone 17 is promoted by the rotating blades 85 and the vortex is kept in continuous rotation by the rotation of mantle 15' . After separation of the fibres into foam column 20, the fibre-depleted water forms a water ring in pump rotor 87 and is pumped through pump housing 88 to outlet 28.

For improving the control of a papermachine, as described in the above mentioned patent application WO 93/23612 by the same inventor, material passing the wire should be returned to the process, i. e. to the papermachine head box, as fast as possible.

Fig. 9 represents a system according to said patent applica¬ tion , which includes a combination of a fibre separation unit 10 according to the present invention and foam extinguishing apparatuses 60, 30, which combination is referred to herein as fibre recovery unit 80.

Papermaking stock and its components are fed from stock pre¬ paration 70 through feeding line 72 to stock mixer 74, where it is diluted by back water flows fed by deaeration pumps 86 connected to the dewatering units of the former, said pumps favorably being of a type disclosed in co-pending patent appli¬ cations WO 93/23135 or Fl 935853, by the same inventor.

A fibre suspension of extinguished foam from outlet 44 of fibre recovery unit 80 is also brought to the mixer 74. The system is planned and dimensioned so that the stock is diluted in mixer 74 to a consistency suitable for subsequent sorting in centrifugal cleaner 75, or favorably less than 1, 5% fibre content. The centrifugal cleaner 75 is preferably a cleaner according to co-pending patent application WO 93/23610 by the same inventor and included herein by reference. Said cleaner has an active rotor for maintaining the rotation. The reject, separating in the cleaner, is diluted in multiple stages 89, each stage being fed by an air separating pump 86.

From the centrifugal cleaner 75, the cleaned stock is brought forward to a pressurized screen 79, where the reject is preferably diluted by back water flow 84, fed by one or more air separating pumps 86. The screen is preferably a screen according to co-pending patent application WO 93/23609 by the same inventor and included herein by reference. Cleaner 75 as well as screen 79 are characterized in that they utilize an internal dilution of the reject and accomplish the cleaning of stock without recycling of rejects and without multiple cleaning stages.

From the pressurized screen 79, the stock is brought to a headbox 90, from where the diluted stock flows onto a moving wire 92, on which a paper web is formed while water separating from the fibre material drains through the wire. Under the wire 92 there are means for conducting backwater passed through the wire to the air separation pumps 86.

A part of the fibre material coming from headbox 90 is transported with the water passing through the wire 92. The amount of said fibre material depends on the draining speed, on the characteristics of the fibre material and on the progress of sheet forming, so that the highest fibre con¬ sistency is in the water removed closest to the headbox 90. After this, the fibre consistency of the water passing the wire gradually decreases, but due to the suction applied, it may increase again in the final stages of dewatering at suction boxes 98 and suction roll 99. After suction box 99 all water to be removed on the wire has been removed.

The paper web 100 leaving wire 92 has a higher dry substance consistency than the dry substance consistency of stoc arriving from stock preparation 70 along stock feeding pipe 72, whereby an excess of water arises, and is removed throug excess water line 103. The removed water can be conducted through dilution water line 104 to couch pit 71 and through water outlet 105 to be reused in stock preparation 70 or it may be discharged as effluent. In order to return the fibres contained in the water passing in excess water line 103 as fast as possible to head box 90 said fibres are removed in fibre recovery unit 80 and returned to headbox 90 via mixer 74.

For the function of the papermachine it is essential that an accurately controlled flow volume is fed into headbox 90. The water circulations forms a closed system, into which stock is fed only through stock line 72, and from which, in addition to paper web 100, is removed only fibre-depleted water through excess water line 103. The process equilibrium of said system has to be maintained by controlling the amount of fibre- depleted water leaving through excess water line 103. This can be done by controlling the amount of fibre-depleted water returning to the process together with fibre suspension separated from extinguished foam through bypass 68 and foam pipe 13 by means of control-valve 66 of Fig. 6.

When planning the process, the air separation pumps feeding fibre separation apparatus 10 are chosen so that during all operating conditions, they produce enough backwater for maintaining a sufficient discharge flow through excess water line 103. Hereby it is also favorable to adjust the fibre consistencies of the waters draining through the wire, so that waters containing as little fibres as possible are brought to fibre recovery unit 80. In the process shown by Fig. 9 fibre- containing water is fed by air separation pumps 86 to foam extinguishing apparatus 60 and to the foam separation nozzles of the rotor of foam extinguishing apparatus 30. Alternative¬ ly, fibre-depleted water from by-pass 68 may be used, whereby the flow through cyclone 10 must be correspondingly increased.

The above description shows that with a fibre separation apparatus according to the invention, fibres in the back-water can be quickly and efficiently separated and returned to the process, whereby the controllability of the process will be improved and the amount of fibres and fillers lost will decrease.

It should be noted that the present invention is not limited to the embodiments presented above, since the persons skilled in the art can easily find alternative embodiments based on the previous description. It is, for instance, conceivable to substitute numerous small nozzles or pipes for the porous part of the mantle, whereby no separate gas chamber is needed. Further it is conceivable that no gas chamber is needed when a sufficient vacuum for sucking air from the environment through the porous part of the mantle is applied, whereby the con¬ struction of the cyclone can be simplified.

Claims

1. A process for separating fibres from a fibre-containing water by means of continuous flotation, c h a r a c t e ¬ r i z e d in that

- s aid fibre-containing water is caused to rotate in a vortex inside a mantle shaped essentially as a body of revolution,

- finely distributed gas is introduced into said vortical water,

- a resulting foam, enriched with fibres, is caused to move toward the center of said vortex, and

- fibre-enriched foam is discharged from said fibre-enriched foam column generated in said center of said vortex, said fibre-enriched foam preferably being trans ferred to foam extinguishing.

2. The process according to claim 1, wherein said fibre- containing water is caused to rotate in a vortex by feeding water tangentially against the inner surface of said mantle at a high speed.

3. The process according to claim 1 or 2, wherein said fibre- containing water is caused to form a vortex by means of a rotor operating inside said mantle.

4. The process according to any one of the preceding claims, wherein said fibre-containing water is rotated in a vortex by rotating said mantle.

5. The process according to any one of the preceding claims, wherein finely distributed gas is dispersed through a gas permeable part of said mantle into said water rotating in a vortex inside said mantle.

6. The process according to any one of the preceding claims, wherein said foam is transferred into a foam extinguishing apparatus for extinguishing said foam and for recovering the resulting fibre suspension.

7. The process according to claim 6, wherein an air outlet of said foam extinguishing apparatus is connected to a vacuum source in order to apply vacuum to said foam column in said mantle and to said foam extinguishing apparatus.

8. An apparatus for separating fibres from a fibre-containing water by means of flotation, which apparatus (10) comprises means for feeding fibre-containing water into said apparatus, means for feeding gas into said water for the forming of fibre- enriched foam, and means for discharging fibre-depleted water and fibre-enriched foam from said apparatus (10), c h a r a c t e r i z e d in that said apparatus (10) com¬ prises a mantle (15; 15' ) shaped essentially as body of revo¬ lution and means (12; 85) for causing said fibre-containing water to rotate in a vortex on the inner surface of said mantle (15; 15' ), and that said means for feeding gas into said water comprise a gas distribution part (18) of said mantle (15; 15' ) for feeding finely distributed gas into the vortical water, and that said means (13) for discharging fibre- enriched foam extend into the area of a foam column generated in the center of said mantle (15; 15' ), said discharging means (13) preferably being connected to a foam extinguishing apparatus (30; 60).

9. The apparatus according to claim 8, wherein said mantle (15; 15' ) is essentially formed as a cylinder or a truncated cone and said means for causing said vortex comprises a spiral inlet head (12) accelerating the flow speed.

10. The apparatus according to claim 9, wherein is a vortex stabilizer (26) is provided in said apparatus (10) at the end opposite to the inlet (12) of said apparatus (10), whereby said stabilizer (26) and said mantle (15) between themselves define an annular slot (27) for discharging fibre-depleted water from said apparatus (10).

11. The apparatus according to any one of the preceding claims 8 to 10, wherein said mantle (15, 15' ) has a gas permeable porous part (18) for feeding finely distributed gas into said mantle, said part (18) being surrounded by a gas chamber (19) connected to a gas inlet (22).

12. The apparatus according to any one of the preceding claims 8 to 11, wherein a center piece (29) is provided at the same end of said apparatus (10) as inlet (12), for promoting a vortical motion, the wall of a foam pipe (13) at the outlet end of said apparatus (10) defining a vortex stabilizer.

13. The apparatus according to claim 8, 9 or 11, wherein said mantle (15' ) is rotatable.

14. The apparatus according to any one of the preceding claims 8 to 13, wherein said apparatus (10) comprises a rotor (50; 85) with blades for rotating said water.

15. The apparatus according to any one of the preceding claims 8 to 14, wherein said foam discharging means (13) are connected to a foam extinguishing device (30) having a circular foam extinguishing shell (38) shaped in cross-section essentially as a truncated cone or funnel, and a rotatable rotor (32) with blades and, at the wider end thereof, an out¬ let (44) for fibre suspension.

16. The apparatus according to claim 15, wherein means (42) for spraying a fluid are provided between the blades of said rotor (32) for promoting the extinguishing of foam.

17. The apparatus according to any one of the preceding claims 8 to 14, wherein an extension of said foam discharge pipe (13) constitutes a foam extinguishing device (60) com- prising nozzles (62) for spraying liquid.

18. The use of an apparatus according to any one of the pre¬ ceding claims 8 to 17 for the recovery of fibres from excess backwater in the back-water circulation of a papermachine, and for the fast separation and returning of said fibres to the fibre process.