US20060254732A1

US20060254732A1 - System and method for removing foreign particles from an aqueous fibrous suspension

Info

Publication number: US20060254732A1
Application number: US11/431,633
Authority: US
Inventors: Juergen Dockal-Baur; Harald Selder
Original assignee: Voith Paper Patent GmbH
Current assignee: Voith Patent GmbH
Priority date: 2005-05-12
Filing date: 2006-05-11
Publication date: 2006-11-16
Also published as: EP1728918A3; JP2006316400A; EP1728918A2

Abstract

A method and system is used for removing foreign particles, e.g., printing ink particles, from an aqueous fibrous suspension, in particular recovered paper suspension. The method includes dividing the suspension in a fractionation into a fine fraction, in particular short-fiber fraction, and a coarse fraction, in particular long-fiber fraction. Both of the fractions thus formed are then treated in respectively differently operated flotations in order to carry away the foreign particles in the flotation foam. The parameters and apparatuses used of the two flotations can be optimally adjusted thereby to the conditions present in the individual fractions.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 of German Patent Application No. 10 2005 021 929.2, filed on May 12, 2005 and German Patent Application No. 10 2005 060 476.5, filed Dec. 17, 2005, the disclosures of which are expressly incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a method for removing foreign particles from an aqueous fibrous suspension and, more particularly, to a method for removing foreign particles in a recovered paper suspension with the aid of several selective flotations.
2. Discussion of Background Information
Methods can be used to eliminate at least part of the undesirable solid particles, e.g., foreign particles, suspended in a paper fiber suspension. A typical method is the treatment of an aqueous fibrous suspension obtained from printed recovered paper, in which suspension of the printing ink particles are already released from fibers so that they can be floated off.
The flotation process utilizes the differences between fibrous paper stock and undesirable solid particles such that the fiber stock remains in the fibrous suspension on account of its rather hydrophilic nature, while the targeted solid particles are hydrophobic and therefore move into the foam together with the air bubbles. In addition to the printing ink particles, there is also a plurality of other substances that are hydrophobic and thus can be separated from the fiber stock through flotation. In particular, such materials are adhesives, fine synthetic material particles and perhaps also resins.
In the flotation process, fibers are separated from impurities by “selective flotation.” The term “flotation de-inking,” which is also used, is normally used for the removal of printing ink particles and more generally for the selective flotation of impurities out of fibrous suspensions.
Methods with two consecutive flotation steps between which a mechanical treatment is carried out are widely used. For example, such methods are described in the article by Herbert Britz “Flotationsdeinking—Grundlagen und Systemeinbindung,” Wochenblatt für Papierfabrikation 10, 1993, pages 394-401. This method describes using first a “Flotation I” then a high-consistency dispersion for its accepted stock and then a “Flotation II” to remove the printing inks detached from the fiber during dispersion. Such methods are very effective but relatively complex, since each partial step contains a complete selective flotation for the entire fibrous paper stock.
DE 102 56 519-A1 shows a method in which the entire fiber stock is first floated (flotation 1). Then a fiber fractionation takes place. The coarse fraction of this fractionation is dispersed and subsequently floated a second time (flotation 2) together with the fine fraction of the fractionation.

SUMMARY OF THE INVENTION

The invention creates a preparation method with which the most effective possible elimination of the foreign particles can be achieved without excessively high expenditure in terms of equipment and operation being required.
The method according to the invention has the advantage that the flotations respectively treat only one fraction and can be optimally adapted to the special requirements of these fractions.
In an aspect of the invention, a method for removing foreign particles from an aqueous fibrous suspension, in particular recovered paper suspension, with the aid of several selective flotations, is provided. The method includes a fractioning device to form a fibrous fine fraction and a fibrous coarse fraction. The fine fraction and coarse fraction are treated in separate flotations. The separate flotation features of inflow consistency, overflow quantity, dwell time, chemical type, specific chemical quantity, location of chemical addition, specific quantity of the flotation air added, bubble size, number of flotation cells, type of flotation cell, size of the gravitational field, number of reject-related flotation steps or number of accepted stock-related flotation steps are selected differently, individually or in combination with one another.
The method is typically applied to recovered paper suspensions. This starting material contains a mixture of different fibers and impurities. Through the fractionation, a smaller proportion of dyes and/or fillers are transferred from this mixture into the coarse fraction, while the fine fraction contains larger amounts of the floatable foreign particles together with the short fibers. In a fine fraction formed in this manner, the flotation conditions are more favorable than in the coarse fraction. That is, the flotation of the fine fraction can yield a good result even, e.g., with higher consistency, while, e.g., the conventional full stream flotation is carried out with a consistency of approximately 1.3%. When the method according to the invention is applied, the fine fraction can be floated at approximately 1.5% and the coarse fraction at approximately 0.9%. In embodiments, a fine adjustment of these consistency values to the conditions and requirements is possible. Other possibilities for influencing the flotation are provided by variations of the chemical metering.
In another aspect of the invention, a method for removing foreign particles from an aqueous fibrous suspension using selective flotations is provided. The method comprises fractioning the suspension to form a fibrous fine fraction and a fibrous coarse fraction. The fine fraction and coarse fraction are treated in separate flotations.
In embodiments, the flotation effects of which differ in that in the separate flotations the features of inflow consistency, overflow quantity, dwell time, chemical type, specific chemical quantity, location of chemical addition, specific quantity of flotation air added, bubble size, number of flotation cells, type of flotation cell, size of gravitational field, number of reject-related flotation steps or number of accepted stock-related flotation steps are selected differently, individually or in combination with one another.
In embodiments, the fine fraction and the coarse fraction are enriched with long fibers. The flotation treating the fine fraction is a short-fiber flotation, and the flotation treating the coarse fraction is a long-fiber flotation. The fine fraction is enriched with fines and the coarse fraction is enriched with short fibers and long fibers.
In embodiments, the fractionation is carried out in a pressurized screen and the flotation treating the coarse fraction is operated with a consistency that differs from the flotation treating the fine fraction by at least approximately 0.5%. The flotation treating the coarse fraction is carried out with a consistency of no more than 1.5% and the flotation treating the fine fraction is carried out with a consistency of no more than approximately 2.5%. More specifically, the flotation treating the coarse fraction is carried out with a consistency between approximately 0.7% and 1.3%, and the flotation treating the fine fraction is carried out with a consistency of between approximately 1.5% to 2%.
The dwell time of the suspension in the flotation treating the coarse fraction is adjusted differently by at least approximately 10% from the dwell time in the flotation treating the fine fraction. More specifically, the dwell time of the suspension in the flotation treating the coarse fraction is adjusted differently by at least approximately 20% from the dwell time in the flotation treating the fine fraction.
In embodiments, in the flotation treating the coarse fraction, the specific quantity of fed flotation air based on volume flow is adjusted differently by at least approximately 10% from in the flotation treating the fine fraction. More specifically, in the flotation treating the coarse fraction, the specific quantity of the fed flotation air based on the volume flow is adjusted differently by at least approximately 20% from in the flotation treating the fine fraction.
In embodiments, an average bubble size in the flotation treating the coarse fraction is adjusted differently from the flotation treating the fine fraction, wherein the difference is at least approximately 10%. More specifically, the average bubble size in the flotation treating the coarse fraction is adjusted differently from in the flotation treating the fine fraction, wherein the difference is at least approximately 20%.
In embodiments, the gravitational field prevailing in the flotation treating the coarse fraction is adjusted differently from that in the flotation treating the fine fraction. In the flotation treating the coarse fraction, a different number of reject-related flotation steps is used than in the flotation treating the fine fraction. In the flotation treating the coarse fraction, a different number of accepted stock-related flotation steps is used than in the flotation treating the fine fraction. In the flotation treating the coarse fraction, a different overflow proportion is formed than in the flotation treating the fine fraction, wherein the difference is at least approximately 20%.
In embodiments, before the flotation, the coarse fraction is thickened, dispersed and diluted again. The dispersing is carried out in a disk disperger or kneader pulper with a consistency between approximately 15% and 35%. The dispersing is carried out through compression milling.
In embodiments, the specific work transferred during the dispersing is between approximately 30 and 120 kWh/t. More specifically, the specific work transferred during the dispersing is approximately 60 kWh/t.
In embodiments, the fractionation is carried out with a pressurized screen that is equipped with at least one wire. The openings in the at least one wire are slots that have a slot width between approximately 0.08 and 0.3 mm. More specifically, the slot width is between approximately 0.1 to 0.15 mm.
In embodiments, the openings in the at least one wire are holes. The openings in the at least one wire are round holes that have a diameter between approximately 0.1 mm and 2 mm. More specifically, the round holes have a diameter between approximately 0.3 mm to 1 mm.
In embodiments, the fractionation is carried out in a worm press.
In embodiments, the fractionation is carried out with a consistency between approximately 0.6% and 4%. The fractionation is carried out in hydrocyclones with a consistency between approximately 0.1% and 2%. More specifically, the fractionation is carried out in hydrocyclones with a consistency between approximately 0.3%-0.7%.
In embodiments, apart from the two flotations, no other flotation is carried out to remove foreign particles from the fibrous suspension.
In another aspect of the invention, a method of removing foreign particles from an aqueous fibrous solution comprises obtaining fine fraction and course fraction. The fine fraction is mixed with air and brought to a first flotation. The fine fraction is cleaned by removing flotation foam from the fine fraction and the flotation foam is guided into a reject processing. The overflowing coarse fraction is diluted with water and brought to a second flotation. The diluted coarse fraction is cleaned in the second flotation by removing the flotation foam from cleaned accepted stock.
In the embodiments described herein, process parameters are modified and selected differently, individually or in combination with one another for the first flotation and the second flotation. In embodiments, the process parameters include at least one of inflow consistency, overflow quantity, dwell time, chemical type, specific chemical quantity, location of chemical addition, specific quantity of flotation air added, bubble size, number of flotation cells, type of flotation cell, size of gravitational field, number of reject-related flotation steps and number of accepted stock-related flotation steps. The consistency is a thickening of the coarse fraction between approximately 15% and 25%, and the diluting is to approximately 0.5% to 1% consistency.
In another aspect of the invention, a system for removing foreign particles from an aqueous fibrous suspension comprises a fractionation configured to separate fine fraction and coarse fraction from the aqueous fibrous suspension. The system also includes a first configuration for processing the fine fraction and a second configuration for processing the coarse fraction. The first configuration comprises a first flotation device configured to clean foreign particles from the fine fraction. The second configuration comprises a thickening apparatus configured to thicken the coarse fraction and a dispersion apparatus configured to detach foreign particles adhering to fibers in the coarse fraction. The second configuration further comprises a dilution apparatus through which addition of water is provided to the course fraction, and a second flotation device configured to clean off floatable impurities from the diluted course fraction.
In embodiments, the fractionation is a wet machine equipped with a screen basket or a washing device. The thickening device is a worm gear. The thickening device thickens the coarse fraction to a higher consistency between approximately 15% and 25%.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:
FIG. 1 shows a diagram of the method according to the invention;
FIG. 2 shows, diagrammatically, a system according to the invention;
FIG. 3 shows an embodiment in accordance with the invention; and
FIG. 4 shows, diagrammatically, a system for implementing the method shown in FIG. 3.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.
FIG. 1 shows a schematic diagram in which the aqueous fibrous suspension S is first fed to a fractionation (fractioning device) 3. With such a fractionation 3, the fibers, which are significantly longer, possibly also stiffer than the other fibers, are guided into the coarse fraction G, while the short fibers, fines, fillers and fine foreign particles are guided into the fine fraction F. In this embodiment, the liquid, mainly water, flows with both fractions. Shifts in the consistency can thereby result, normally such that the fine fraction F has a lower consistency than the coarse fraction G.
The fractionation 3 can also be carried out such that all the fibers, or a very large part of them, reach the coarse fraction G. The fine fraction F then contains few fibers (also few short fibers), but a large part of the organic and inorganic fines, including the foreign particles to be floated. The proportion of the fibers, as a rule, is determined as a residue of the wire R 100 according to Bauer-McNett (laboratory method according to TAPPI standard T 233 which should be known to those of skill in the art). Fines accrue in the same analysis as throughput of the wire.
As FIG. 1 shows, the fine fraction F is guided into a flotation 1 and is cleaned of foreign particles in a known manner. This flotation 1 can also be called a short-fiber flotation, since its operating parameters are adapted to a suspension particularly enriched with short fibers and fines. The processes with the flotation 1 are known such as, for example, by forming a flotation foam R1 which contains a large part of the foreign particles, and a cleaned accepted stock A1 with the predominant part of the fibers. The flotation foam R1 is removed and guided into the reject processing 8.
The coarse fraction G of the fractionation 3 is diluted with water W and cleaned in the flotation 2, which can also be called a long-fiber flotation, from which a cleaned accepted stock A2 is obtained. A flotation foam R2 for carrying away the foreign particles is also formed. The flotation foam R2 removed and guided into the reject processing 8′.
In order to carry out the method according to the invention in a optimal and therefore particularly economic and effective manner, operating parameters and/or the system used for flotation respectively to the suspension to be floated, e.g., the fine fraction F and the coarse fraction G, can be modified. One parameter in flotation is the intake consistency, i.e., the solids content, with which the suspension is fed to the flotation system. The overflow quantity, based on the throughput amount of suspension, also has an influence on the effectiveness, and in particular on the cleanness of the accepted stock. A large overflow quantity improves the accepted stock, but leads to higher losses or to greater expenditure in the further steps concerning reject. A greater dwell time of the suspension in the flotation area can, at least within certain limits, improve the flotation effect or adjust it to more difficult materials.
With flotation chemicals, the type and the specific chemical quantity based on the fiber quantity can be varied. The cost of the chemicals, which plays a role in the operating costs of a flotation system, can thereby be minimized. Thus, for example, adjustments can be made to the greater toughness of a long-fiber suspension compared to a short-fiber suspension through the quantity and type of the chemicals. A targeted change is also possible through the choice of the location for adding the chemicals. As is known by those of skill in the art, chemicals can, at least, partially be added during the slushing as well as directly before the flotation system.
The quantity and bubble size of the flotation air fed also influence the flotation effect. In accordance with the invention, these parameters are adjusted in particular to the size of the particles to be floated as discussed herein. A larger number of flotation cells can intensify their effect while parameters otherwise remain the same as discussed herein.
As is known, there are also differences in the type of flotation cells, e.g., whether it is a flotation tower with suspension guidance flowing essentially vertically or a horizontal flotation cell. While most flotation cells are operated in the gravitational field, there is also the possibility of intensifying the gravitational field, e.g., through eddies or cyclones. In many cases, the flotation is carried out again, once or several times, on the reject side of a flotation system, which is usually called the reject-related further flotation step. Thus, for example, the overflow quantity can be increased and a good yield of the entire flotation system can be achieved. Several flotation steps can also flow through in succession, i.e., one after the other, on the accept side. In many cases, it will be sufficient to change one of the parameters mentioned, but it is possible in accordance with the invention to change several of these parameters to attain the desired advantages according to the invention.
To clarify the functional diagram shown in FIG. 1, FIG. 2 shows a detailed diagrammatic view of the system. As shown in FIG. 2, a wet screening machine of the pressurized screen type is used for the fractionation 3, where the screen is equipped and operated according to the requirements of a fractionation. The screen is therefore not intended to be used for separating contaminants with which, if possible, all the fibers reach the throughput. The parameters necessary for fractionation in the pressurized screen 7 are known such as, for example, shape and size of the wire openings, overflow rate and wire clearing. In contrast to contaminant removal (“screening”), with fractionation thus also the overflow comprises mostly fibers.
In accordance with the embodiments of the invention, the fibrous suspension S is pumped into the interior of the housing of a pressurized screen and fractionated with the aid of a screen basket 7. The parameters necessary for fractionation in the pressurized screen are known such as, for example, shape and size of the wire openings, consistency of the suspension, overflow rate and type of wire clearing. Typical wire openings of the screen basket 7 are round holes in the range between approximately 0.2 and 0.5 mm or slots between approximately 0.1 and 0.3 mm. The division of the fiber stock into coarse and fine fraction can be adjusted through the selection of the devices and operating conditions for realizing the fractionation 3.
Alternatively, the fractionation 3 can also be operated through the use of a washing device. The filtrate of the washing device is the fine fraction F. Such washing devices for fiber stocks are known and are used not only to separate the water from the solid (filter), but also fractionate the solid, itself. A particularly suitable technical embodiment and the preferred parameters are described, for example, in patent specification DE 30 05 681, which is incorporated herein by reference in its entirety. Devices that work in a similar manner with two circulating wire belts are also known such as, for example, as shown by EP 0 341 913, which is incorporated herein by reference in its entirety. However, as is known, fibers can also be fractionated in hydrocyclones, whereby a consistency between approximately 0.3% and 0.7% is particularly favorable.
Directly before entry into a flotation system, the fine fraction F of the fractionation 3 is mixed with air L and then brought to flotation. The flotation system can have several steps, e.g., two, on the reject side. The formed flotation foam R1 is removed and guided into the reject processing 8. Depending on the system, the reject processing 8 can be carried out in one or more flotation steps, for which a number of different switching possibilities are known.
After dilution with water W, the overflow of the fractionating pressurized screen, e.g., the coarse fraction G, is directed into the flotation 2. As has already been discussed, the flotation of the coarse fraction G is conducted, in general, under more difficult conditions compared to the fine fraction F. One of the reasons for this is the tendency of the long fibers to form flakes that impede the floating out of foreign particles. The viscosity of the coarse fraction is also higher than that of the fine fraction. A reduction of the consistency, e.g., to values under approximately 1.3% improves the flotation result a great deal. Since the coarse fraction G is only a partial stream, the dilution causes a relatively low strain on the overall process (stock balance, water circuits).
According to FIG. 3, the already disintegrated recovered paper A is cleaned of coarse and medium-sized impurities through a screening 9. Perforated screens and hydrocyclones in particular are suitable for this processing step. The fibrous suspension S thus formed is divided by fractionation 3 into fine fraction F and coarse fraction G. In many cases it is useful to conduct a dispersion 4 before carrying out a flotation, which is shown in FIGS. 3 and 4. Through the thickening 5, the coarse fraction G gains a higher consistency, preferably between approximately 15% and 25%, then undergoes a dispersion 4 and subsequently a dilution 6, e.g., to approximately 0.5% to 1% consistency. As is known, the purpose of a dispersion 4 is the detachment of foreign particles adhering to the fibers. Possibly other effects that are achievable with dispergers, such as, e.g., slushing the residual fiber bundles, can also be connected therewith. The dispersed and diluted stock then reaches the flotation 2.
FIG. 4 shows a more detailed representation of a system used for implementing the method. The fibrous suspension S is first fed to a fractionation 3 that also takes place with the aid of a pressurized screen equipped with a screen basket 7. As is known, however, fibers can also be fractionated in hydrocyclones or wire presses. The fine fraction F, e.g., the throughput through the screen basket 7, reaches the flotation 1 in which a cleaned accepted stock A1 and a flotation foam R1 are formed.
The consistency of the coarse fraction G from the fractionation 3 is increased by thickening to a value between approximately 15% and 25%. To this end, a worm press 19 may be used as one example; however, as is known, there are also other thickening apparatuses possibly working in a multi-stage manner. The filtrate 10 from this thickening can be used to form the fibrous suspension S. The thickened stock 11 then reaches a disk disperger 18 via a conveyor screw system (not shown), in which the disperger, e.g., dispersion tools provided with teeth are moved relative to one another at a close distance, causes a dispersion of the high-consistency stock through high shear forces. Alternatively, a kneader pulper can also be used for this processing step.
Normally it is expedient to heat the high-consistency stock provided for dispersion, e.g., to a temperature between approximately 800 and 95° C. To this end, a direct feed of steam D into the disk disperger 18 can take place. Another possibility for the dispersion of the coarse fraction G lies in the use of a compression refiner that works according to a method that is described, e.g., in DE-A-102 36 962 (which is incorporated by reference in its entirety). This method yields a very good dispersion without fiber damage. Compared to the disk disperger or kneader pulper, this method can be carried out at lower consistency, resulting in savings in terms of equipment and energy. The mechanical work that is transferred to the fiber stock during the dispersion is usually adjusted to a value between approximately 30 and 120 kWh/t, preferably approximately 60 kWh/t.
After the dispersion, a dilution 6 occurs through the addition of water W, e.g., in a vat. The fibrous suspension thus prepared is subsequently cleaned of floatable impurities in the flotation 2 with good results. In many cases, the accepted stock A1 of the flotation 1 and the accepted stock A2 of the flotation 2 can be mixed and further processed in a vat 12. As a rule, it is no longer necessary to provide a further flotation; instead the invention makes it possible to save on further flotations, since an already fully satisfactory flotation result can be achieved through the flotations 1 and 2 specifically adapted to the individual fractions.
It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.

Claims

1. A method for removing foreign particles from an aqueous fibrous suspension using selective flotations, the method comprising:

fractioning the suspension to form a fibrous fine fraction and a fibrous coarse fraction; and

treating the fine fraction and coarse fraction in separate flotations.

2. The method according to claim 1, wherein the fine fraction and the coarse fraction are enriched with long fibers.

3. The method according to claim 2, wherein the flotation treating the fine fraction is a short-fiber flotation, and the flotation treating the coarse fraction is a long-fiber flotation.

4. The method according to claim 1, wherein the fine fraction is enriched with fines and the coarse fraction is enriched with short fibers and long fibers.

5. The method according to claim 1, wherein the fractioning is carried out in a pressurized screen and the flotation treating the coarse fraction is operated with a consistency that differs from the flotation treating the fine fraction by at least approximately 0.5%.

6. The method according to claim 1, wherein the flotation treating the coarse fraction is carried out with a consistency of no more than approximately 1.5% and the flotation treating the fine fraction is carried out with a consistency of no more than approximately 2.5%.

7. The method according to claim 1, wherein a dwell time of the suspension in the flotation treating the coarse fraction is adjusted differently by at least approximately 10% from the dwell time in the flotation treating the fine fraction.

8. The method according to claim 1, wherein, in the flotation treating the coarse fraction, a specific quantity of fed flotation air based on volume flow, is adjusted differently by at least approximately 10% from in the flotation treating the fine fraction.

9. The method according to claim 1, wherein an average bubble size in the flotation treating the coarse fraction is adjusted differently from the flotation treating the fine fraction, wherein the difference is at least approximately 10%.

10. The method according to claim 1, wherein a gravitational field prevailing in the flotation treating the coarse fraction is adjusted differently from that in the flotation treating the fine fraction.

11. The method according to claim 1, wherein, in the flotation treating the coarse fraction, a different number of reject-related flotation steps is used than in the flotation treating the fine fraction.

12. The method according to claim 1, wherein, in the flotation treating the coarse fraction, a different number of accepted stock-related flotation steps is used than in the flotation treating the fine fraction.

13. The method according to claim 1, wherein, in the flotation treating the coarse fraction, a different overflow proportion is formed than in the flotation treating the fine fraction, wherein the difference is at least approximately 20%.

14. The method according to claim 1, wherein before the flotation of the coarse fraction, the coarse fraction is thickened, dispersed and diluted again.

15. The method according to claim 14, wherein the dispersion is carried out in a disk disperger or kneader pulper with a consistency between approximately 15% and 35%.

16. The method according claim 14, wherein the dispersion is carried out through compression milling.

17. The method according to claim 14, wherein specific work transferred during the dispersing is between approximately 30 and 120 kWh/t.

18. The method according to claim 1, wherein the fractionation is carried out with a pressurized screen that is equipped with at least one wire.

19. The method according to claim 18, wherein openings in the at least one wire are slots that have a slot width between approximately 0.08 and 0.3 mm.

20. The method according to claim 18, wherein the openings in the at least one wire are holes.

21. The method according to claim 20, wherein the openings in the at least one wire are round holes that have a diameter between approximately 0.1 mm and 2 mm.

22. The method according to claim 1, wherein the fractionation is carried out in a worm press.

23. The method according to claim 1, wherein the fractionation is carried but with a consistency between approximately 0.6% and 4%.

24. The method according to claim 1, wherein the fractionation is carried out in hydrocyclones with a consistency between approximately 0.1% and 2%.

25. The method according to claim 1, wherein, apart from the two flotations, no other flotation is carried out to remove foreign particles from the fibrous suspension.

26. The method according to claim 6, wherein the flotation treating the coarse fraction is carried out with a consistency between approximately 0.7% and 1.3%, and the flotation treating the fine fraction with a consistency of between approximately 1.5% to 2%.

27. The method according to claim 7, wherein the dwell time of the suspension in the flotation treating the coarse fraction is adjusted differently by at least 20% from the dwell time in the flotation treating the fine fraction.

28. The method according to claim 8, wherein, in the flotation treating the coarse fraction, the specific quantity of the fed flotation air based on the volume flow, is adjusted differently by at least approximately 20% from in the flotation treating the fine fraction.

29. The method according to claim 9, wherein the average bubble size in the flotation treating the coarse fraction is adjusted differently from in the flotation treating the fine fraction, wherein the difference is at least approximately 20%.

30. The method according to claim 17, wherein the specific work transferred during the dispersing is approximately 60 kWh/t.

31. The method according to claim 19, wherein the slot width is between approximately 0.1 mm to 0.15 mm.

32. The method according to claim 21, wherein the round holes have a diameter between approximately 0.3 mm to 1 mm.

33. The method according to claim 24, wherein the fractionation is carried out in hydrocyclones with a consistency between approximately 0.3% to 0.7%.

34. The method according to claim 1, wherein, within separate flotations, the flotation effects differ in that in the separate flotations the features of inflow consistency, overflow quantity, dwell time, chemical type, specific chemical quantity, location of chemical addition, specific quantity of flotation air added, bubble size, number of flotation cells, type of flotation cell, size of gravitational field, number of reject-related flotation steps or number of accepted stock-related flotation steps are selected differently, individually or in combination with one another.

35. A method, comprising:

obtaining a fine fraction and a course fraction from an aqueous fibrous solution;

mixing the fine fraction with air and bringing the mixed air and fine fraction to a first flotation;

cleaning the fine fraction in the first flotation by removing flotation foam from the fine fraction and guiding the flotation foam into a reject processing;

diluting overflowing coarse fraction with water and bringing the diluted course fraction to a second flotation; and

cleaning the diluted coarse fraction in the second flotation by removing flotation foam from cleaned accepted stock obtained from the second flotation,

36. The method according to claim 35, wherein process parameters are modified and selected differently, individually or in combination with one another for the first flotation and the second flotation.

37. The method according to claim 36, wherein the process parameters include at least one of inflow consistency, overflow quantity, dwell time, chemical type, specific chemical quantity, location of chemical addition, specific quantity of flotation air added, bubble size, number of flotation cells, type of flotation cell, size of gravitational field, number of reject-related flotation steps and number of accepted stock-related flotation steps.

38. The method according to claim 37, wherein the consistency is a thickening of the coarse fraction between approximately 15% and 25%.

39. A system for removing foreign particles from an aqueous fibrous suspension, comprising:

a fractioning device configured to separate fine fraction and coarse fraction from the aqueous fibrous suspension; and

a first configuration for processing the fine fraction and a second configuration for processing the coarse fraction, wherein:

the first configuration comprising:

a first flotation device configured to clean foreign particles from the fine fraction;

the second configuration comprising:

a dispersion apparatus configured to detach foreign particles adhering to fibers in the coarse fraction;

a dilution apparatus through which water is added to the course fraction; and

a second flotation device configured to clean floatable impurities from the diluted course fraction.

40. The system according to claim 39, wherein the fractioning device is one of a wet machine equipped with a screen basket and a washing device.

41. The system according to claim 39, wherein the second configuration comprise a thickening apparatus configured to thicken the coarse fraction.

42. The system according to claim 38, wherein the thickening device thickens the coarse fraction to a higher consistency between approximately 15% and 25%.