CA2278610A1

CA2278610A1 - Method and apparatus for treatment of fiber suspension

Info

Publication number: CA2278610A1
Application number: CA002278610A
Authority: CA
Inventors: Erkki Hietala; Ismo Hamalainen
Original assignee: Individual
Current assignee: Andritz Oy
Priority date: 1997-11-11
Filing date: 1998-11-11
Publication date: 1999-05-20
Also published as: FI974192A0; FI974192A; ID22428A; FI103734B; FI103734B1; WO1999024662A1

Abstract

The present invention relates to a method and apparatus for treatment of fiber suspensions in the pulp and paper industry. The invention relates in particular to intensifying the so-called post-screening or secondary screening of bleached pulp or pulp in the process of being bleached. It is characteristic of the method of the invention for treating fiber suspension in the so-called secondary screening, during which impurities are removed from the pulp by means of a pressure screen, i.e. a so-called primary screen, and the impurities are further treated at least in one screening stage, that the reject from the pressure screen mentioned is treated in one treatment step or several treatment steps at least one of which is a reverse hydrocyclone step. The apparatus according to the invention comprising at least a pressure screen (30) and intended for treating fiber suspension in the so-called secondary screening process in an apparatus is characterized in that the apparatus comprises at least one reverse hydrocyclone step (40; 46; 48; 57; 64).

Description

Method and apparatus for treatment of fiber suspension The present invention relates to a method and apparatus for the treatment of fiber suspensions in the pulp and paper industry. The invention relates in particular to intensifying the so-called post-screening or secondary screening of bleached pulp or pulp which is in the bleaching stage.
There are prior art systems used for the corresponding purpose according to which pulp in the bleaching stage is guided to a hydrocyclone plant in which heavier impurities such as sticks, large fiber accumulations, and so on, still remaining in the pulp are separated from the pulp. As is known, a hydrocyclone plant consists of dozens of vortex cleaners, i.e.
hydrocyclones, in which the cleaning effect is brought about by causing the material to be cleaned to circulate at a high speed along the cylindrical or conical wall of the cleaner.
Then the coarse fraction to be rejected is collected by the centrifugal force to the surface of the cyclone and the surface is soon worn out; then the cleaning capacity of that cleaner decreases at first when extra friction rapidly decreases the circulation speed of the material. In the end, a whole is worn through the wall of the cleaner and the material to be cleaned is discharged to the floor of the screen room. Further, the hydrocyclone plant requires quite a large space, mainly due to the large number of cyclones. The use of cyclones further has a third drawback, i.e. a remarkable increase in the volume of the internal circulation waters and the effluent load of the pulp mill. As is known, the consistency of the pulp suspension to be cleaned must be of the order of tenths of a percent in order to enable a hydrocyclone to work efficiently and separate well enough the acceptable fiber fraction from knot particles and sticks.
A further drawback of a hydrocyclone cleaning plant should be mentioned, i.e.
the fact that bleaching plants usually employ washers, the washing boxes of which require a consistency of about 1.2 %, at least 1.0 %, of the pulp to be fed into the boxes. Also the drying machine, which is the subsequent treatment phase in the process following the bleaching, requires the same feed consistency of about 1.0 - 1.2 %. However, a hydrocyclone plant does not work efficiently enough at the consistency mentioned, it requires a consistency of about 0.5 %. This means that pulp must be thickened after a hydrocyclone plant from a consistency of 0.5 % to a consistency of 1.0 -1.2 %.
The drawbacks of the apparatus of the old technology described above may be eliminated by replacing the hydrocyclone plant with, for example, narrow-slot screening whereby the task of dozens of hydrocyclones is taken care of by one single screen. At the same time the thickener required after a hydrocyclone plant is dispensed with. This approach has been described for example in U.S. patents no. 5,470,432 and 5,571,384. The saving of space as well as the reduction of the liquid load of the plant are obvious as there is no need to dilute the suspension to be treated to the consistency required by a hydrocyclone plant but the consistency may be the one required by the washer or the drying machine feed box. Thus the volume of dilution liquid needed is reduced remarkably. The U.S.
patents describe an approach in which the reject from a pressure screen is treated in a hydrocyclone plant and the accept from the hydrocyclone plant is returned to the feed of the pressure screen and the reject is either discharged from the process altogether or forwarded to a previous treatment stage.
A disadvantage or a kind of default of the above U.S. patents is that they do not disclose what happens to the reject of the pressure screen. The publications mention only that the reject is treated further in a hydrocyclone plant and the accept from the hydrocyclone plant is returned for reuse and the reject is discharged. However, practice has proved that the approach according to the invention allows particles to pass both to the accept and to the reject which do not belong there. Thus, it has been necessary to develop this so-called post-screening or secondary screening concept further.
It is known that in conventional screens employing screen drums having either holes or slots, particles are divided into accept or acceptable fraction and reject or rejectable fraction mainly based on their size. Particularly when pressure screens are used, some compressible particles, for example expanded polystyrene bits, are know to be pressed through the openings in the screen plate and thus end up to the accept. The only means of preventing this in screen-type apparatus is to use an adequately small screen aperture, preferably a narrow slot, whereby the pressure prevailing in the screen cannot press the pieces through the aperture. This may be done in a primary screen but at the same time also some of the fiber material which has already been accepted at this stage is entrained to the reject and it must then be recovered in a further screening stage. If a screen must be used in a secondary stage for screening, the screen must have larger screen openings for recovering fiber material; then for example the expanded polystyrene pieces mentioned will pass through the secondary screen as such. Thus, it is almost impossible to separate non-desirable material of for example the type mentioned above from the accepted fiber material.
It is also known that the screening action in a hydrocyclone is based mainly on the specific gravity of the particles whereby in conventional applications {for example separation of sand) heavy particles are rejected and the light fraction is recovered. Thus, also a conventional hydrocyclone accepts light reject, for example expanded polystyrene bits and also other light pieces, such as plastics particles, with the fiber material.
A so-called reverse hydrocyclone disclosed for example in U.S. patent no.

4,155,839 has been developed for solving the problem mentioned. A characteristic feature of the cleaner in question, and of its function in particular, is that it is capable of separating light rejects, for example plastics, and acceptable fiber material from each another efficiently and precisely although the specific gravities of the materials do not differ much. A cleaner of this type works so that it discharges the heavier fiber fraction via its narrower end at the tip of a cone which in the conventional position of the apparatus is the lower end, and the lighter rej ect via a discharge opening located centrally at the top end.
Reversed hydrocyclones have, however, been erected also the other way around, i.e. the tip of the cone upwards. Further, it is typical of reversed hydrocyclones that they may be used to thicken pulp. For example the U.S. patent 4,155,839 mentioned above indicated that at a feed consistency of 1.0 -1.5 % the consistency of the accept was 1.6 - 2.6 %.
A severe problem which may be solved with the method and the apparatus according to the present invention is connected with the so-called secondary screening plant described above which is capable of preventing by means of the state-of the-art narrow slot screening for example plastics pieces from ending up to the end product.
However, since the state-of the-art secondary screening plant comprises a hydrocyclone as a device subsequent to the narrow-slot screen, it accepts also light reject, i.e. for example plastics particles, with the fiber material. The light reject mentioned remains circulating in the screen circulation until it is ground into so small pieces that the narrow-slot screen accepts it. As a consequence the light reject ends up to the end product and in the worst case causes clearly perceivable quality problems.
According to a preferred embodiment of the method and the apparatus of the invention, reject from the so-called primary screen is taken to a pressure screen, and reject from this pressures screen is further treated in a so-called first hydrocyclone step.
The method and the apparatus described above may still be developed further so that accept from the first hydrocyclone step mentioned is taken either to a second, reversed hydrocyclone step or to a pressure screen.
The method and the apparatus described above may still be developed further so that the accept from the reverse hydrocyclone step or the pressure screen mentioned is brought to the feed of the secondary screen and the reject is removed to a second reverse hydrocyclone step.
According to another preferred embodiment of the method and the apparatus of the invention, the reject from the so-called primary screen is introduced into the first hydrocyclone step, the accept from which is treated in the reverse hydrocyclone step, the accept from which is returned to the feed of the primary screen and the reject is removed from the process or guided to a suitable further treatment point.
The methods and the apparatus according to the embodiments described above improve the cleaning result of the screening plant remarkably because different screening apparatus, such as pressure screen, hydrocyclone and reverse hydrocyclone, are used in a 3 0 way to utilize their best properties, only. The apparatus in question provide excellent supplement to each other so that the final reject contains only particles, which really should be rejected.

Further, the apparatus according to the invention is essentially less expensive for the pulp mill to purchase and to maintain; thus, considering also the improved efficiency/accuracy of the screening plant, a superior product is provided compared with both prior art 5 methods and apparatus.
The characteristic features of the method and the apparatus of the invention are disclosed in the appended patent claims.
The invention will be described below more in detail with reference to the accompanying drawing figures of which Fig. 1 illustrates schematically a prior art apparatus;
Fig. 2 illustrates schematically another prior art apparatus;
Fig. 3 illustrates schematically an apparatus according to a preferred embodiment of the invention;
Fig. 4 illustrates schematically an apparatus according to another preferred embodiment of the invention;
Fig. 5 illustrates schematically an apparatus according to a third preferred embodiment of the invention; and Fig. 6 illustrates schematically an apparatus according to a fourth preferred embodiment of the invention.
According to Fig. 1, a prior art arrangement comprises mainly a bleaching tower 2, a subsequent hydrocyclone plant 4, a thickener 6 and a washer, most usually a drum washer 8, and a bleaching tower 10 of the subsequent bleaching stage. Fiber suspension to be treated is discharged from the tower 2 to a pipe line in which the suspension is diluted to a consistency of about 0.5 % and supplied further to the hydrocyclones 4. After the hydrocyclones the accepted material is fed directly to the thickener 6, which thickens the pulp to the feed consistency of the subsequent washer 8. From the thickener 6 the pulp is supplied to the washer and the washed pulp is discharged from the washer at the bleaching consistency of the bleaching tower. The rej ect from the hydrocyclones is removed from the apparatus either to be destroyed or to be guided to a previous treatment stage.
Figure 2 illustrates another prior art arrangement (U.S. 5,571,384) comprising a bleaching tower 20, a screen 22, a hydrocyclone/cleaning plant 24, a washer 26 and a bleaching tower (not illustrated) of the subsequent, usually the last bleaching stage.
According to the invention, the pulp bleached in the tower 20 is discharged from the bleaching tower 20 to a pipeline. The pulp either has been diluted while being discharged or it is diluted in the pipeline to a screening consistency of > about 1.0 - 1.2 %; from the pipeline the pulp is supplied to the screen 22. The accept from the washer 22 is fed directly into the washer 26 and the reject is supplied to a hydrocyclone plant 24, which is remarkably smaller than prior art hydrocyclone plants, at a consistency diluted to be suitable for the cyclones. The accept from the hydrocyclones 24 is recirculated to the feed of the screen 22 mentioned above or directly to the washer 26. The reject in turn is either destroyed or returned to a previous suspension treatment stage.
The apparatus according to a preferred embodiment of the invention illustrated in Fig. 3 comprises a so-called primary screen 30 and the accept from this is directed by prior art means in a so-called main treatment line 30a to the process for further use.
The reject from the primary screen 30, i.e. the coarse fraction, is in this embodiment transported in a line 30r to a second step pressure screen 32, a so-called secondary screen;
the accept from the secondary screen is returned in line 32a to the feed of the primary screen 30, and the reject, the coarser fraction, is directed in line 32r to a first hydrocyclone step 34. In some cases, particularly when the secondary screen has been constructed in the required way, the accept from the secondary screen may be returned to the main line 30a to a location after the primary screen (illustrated with a broken line). The accept, i.e.
the lighter fraction, from the first hydrocyclone step 34 is guided in line 34a to a pressure screen 36 and the reject, i.e. the heavier fraction, is taken in line 34r to a second hydrocyclone step 38. The accept from the second hydrocyclone step 38 is returned in line 38a to the feed of the first hydrocyclone step 34 and the reject is discharged from the process.
The accept from the pressure screen 36, i.e. the fraction which has passed through the screen surface, is guided in line 36a to the feed of the pressure screen 32 of the second screening step and the reject, i.e. the coarser fraction, in line 36r to a reverse hydrocyclone 40. The heavier fraction from the hydrocyclone 40 is recirculated in line 40a to the feed of the pressure screen 36 as the accept from the reverse hydrocyclone step 40, whereas the lighter fraction obtained from the cleaner 40 is removed (line 40r) as reject from the process, or is guided to a suitable further treatment.
The operation of the apparatus illustrated in Fig. 3 is described below in a more detailed fashion by way of an example. In the example case, the screening member serving as the primary screen 30 is a slotted cylinder having a slot width of 0.15 mm. A
cylinder of this kind accepts, or allows passage therethrough of, only fiber material suitable for further use. All larger fiber particles, sand, sticks, and other impurities, such as for example plastics, entrain to the reject. The reject is conveyed to the secondary screen 32 preferably comprising a slotted cylinder having a slow width of 0.18 mm. In principle also this cylinder should accept only acceptable fiber fraction but, only to be sure, the accept from this screen is returned to the feed of the primary screen for rescreening. The reject from the secondary screen is conveyed to the first hydrocyclone step 34 comprising a required number of cleaners connected in series. The task of the first hydrocyclone step is to separate from the reject of the secondary screen, on one hand, the acceptable fiber fraction to the accept and, on the other hand, the heavy fraction, such as sand, to the reject. The heavy reject is treated in the secondary hydrocyclone step 38 in order to recover usable fiber material and to return it to the first hydrocyclone step;
after this almost only sand or other heavy impurities are removed from the step 38 as the reject.
The accept from the first hydrocyclone step 34 is conveyed at a very low consistency (of the order of 0.4. - 0.6 %), which is typical of the accept from cleaners, to the pressure screen 36 which has a very small slot size, for example 0.12 mm. The purpose is to separate from the accept of the first hydrocyclone step almost all the usable fiber fraction and to return it to the feed of the secondary screen. Reject from the pressure screen 36, which may be taken either continuously or intermittently depending on the amount of the reject, is treated in the reverse hydrocyclone 40 the main task of which is to separate the light reject from the fiber-containing material. The fiber flow obtained as the accept from the reverse hydrocyclone step 40 at the tip of the cone is conveyed in a thickened state to the feed of the pressure screen 36 through which the fiber fraction is further taken into use. When slotted cylinders are used in the different screens the slot width of the secondary screen 32 drum is less than 0.25 mm, preferably less than 0.20 mm, and in the screen 36 less than 0.20 mm, preferably less than 0.15 mm.
The apparatus according to another preferred embodiment of the invention illustrated in Fig. 4 comprises, like the one in Fig. 3, a primary screen 30, a secondary screen 32, and a subsequent first and a second hydrocyclone step 34 and 38. In fact, the only difference between the embodiment illustrated in Fig. 3 and the one in Fig. 4 is that the pressure screen 36 of Fig. 3 has been replaced in Fig. 4 with a reverse hydrocyclone step 46, the accept of which, i.e. the heavier fraction, is returned in line 46a to the feed of the secondary screen 32 and the lighter fraction discharged as reject is conveyed in line 46r to a second reverse hydrocyclone step 48. The accept from the second reverse hydrocyclone step is conveyed in line 48a to the feed of the first reverse hydrocyclone step and the reject is discharged (line 48r) from the process. It is possible also in this embodiment, and particularly when the secondary screen has been constructed in the required way, to return the accept from the secondary screen also to the main line 30a to a position after the primary screen (illustrated with a dash line). When slotted cylinders are used in the different screens, the slot width of the cylinder in the primary screen 30 is less than 0.20 mm, preferably about 0.15 mm and the slot width of the cylinder in the secondary screen 32 is less than 0.25 mm, preferably less than 0.20.
The apparatus according to a third preferred embodiment of the invention illustrated in Fig. 5 comprises, like the previous embodiments, a primary screen 30, a secondary screen 32, and a first hydrocyclone step 34. It differs from the previous embodiments in that the accept from the secondary screen 32 is conveyed in line 32a, instead of the feed of the primary screen 32, to a reverse hydrocyclone 57 and the accept, i.e. the heavier fraction, from the hydrocyclone 57 is conveyed in line 57a to the feed of the primary screen. This means that the secondary screen 32 removes large particles to the hydrocyclone step 34 which classifies them according to their specific gravity either to reject (line 34r) or to be returned in line 34a to the feed of the secondary screen 32. The accept of the secondary screen contains light material which is taken to the reverse hydrocyclone 57 in which the light reject (line 57r) is separated from the process and the heavier, the fiber fraction is returned to the feed of the primary screen 30. Also in this embodiment, and particularly when the secondary screen 32 and the hydrocyclone step 34 have been constructed in the required way, the accept from the hydrocyclone step 34 may be recirculated to the main line 30a to a point after the primary screen (illustrated with a dash line).
The apparatus according to a fourth preferred embodiment of the invention illustrated in Fig. 6 comprises a primary screen 30, the reject from which is conveyed by prior art methods in line 30r to a first hydrocyclone step 62, the reject from which is removed by a know method in line 62r from the process or it is conveyed to a suitable further treatment step. The new approach in the apparatus of this embodiment is to convey the accept from the first hydrocyclone step 62 in line 62a to a reversed hydrocyclone step 64 the heavier fraction from which is returned in line 64a to the feed of the primary screen 30 and the lighter fraction is rejected from the process. A second new feature is that the filtrate from the thickening device 66 following the primary screen in the line 30a is treated by flotation in order to recover fiber material.
The invention preferably employs a screen cylinder disclosed in Finnish patent application no. 955724, comprising a plurality of screen wires provided side by side and secured firmly to support wires provided on the accept space side. In the approach of the present invention, narrow slots are preferably used the width of which is of the order of <
0.25 mm, preferably < 0.20 mm, sometimes even nearly of the order of 0.10 mm.
It should be mentioned here that, in addition to cylindrical, the screen element may have the form of a cone, a double cone, or even a plane.
The rotor in the pressure screen mentioned may be for example the rotor illustrated in U.S. patent no. 5,000,842 in which the surface of the rotor cylinder has been provided with so-called projections to generate turbulence which disrupts fiber flocs in the suspension. Also so-called foil rotors may be used in some cases. Further, a screen cylinder either inside or outside the rotor may be used, or even cylinders provided on both sides of the rotor. Further, in some cases, a rotating screen cylinder or cylinders may be used.

As may be understood from the above description, a method and apparatus for screening suspension has been developed which, compared with prior art methods and apparatus, is very user-friendly and advantageous for both the process economy and the energy economy. However, it should be noted that the invention has been described above by 5 way of reference to only a few embodiments thereof although the real scope of protection of the invention is much broader and covers more. Thus, the invention is defined by the appended patent claims, only.

Claims

We claim:

1. A method of treating fiber suspension in the so-called secondary screening, during which a pressure screen, i.e. a so-called primary screen is used to remove from pulp impurities which are further treated at least in one screening stage, characterized in that the reject from the pressure screen mentioned is treated in one treatment step or several treatment steps, at least one of which is a reverse hydrocyclone treatments.

2. A method as recited in claim 1, characterized in that the reject from the primary screen mentioned is treated at first in a pressure screen, a so-called secondary screen, the reject from which is later treated in a reverse hydrocyclone treatment.

3. A method as recited in claim 2, characterized in that the reject from the so-called secondary screen mentioned is treated in a hydrocyclone step and the lighter fraction accepted by the hydrocyclone step is treated in the reverse hydrocyclone treatment mentioned.

4. A method as recited in claim 3, characterized in that the lighter fraction mentioned is treated at first in a pressure screen, the reject from which is conveyed to the reverse hydrocyclone treatment mentioned.

5. A method as recited in claim 3, characterized in that the lighter fraction mentioned is treated directly by a reverse hydrocyclone treatment.

6. A method as recited in claim 1, characterized in that the reject from the primary screen mentioned is conveyed to a hydrocyclone step and the lighter fraction accepted by the hydrocyclone step is conveyed to a reverse hydrocyclone treatment.

7. A method as recited in claim 6, characterized in that the heavier fraction accepted by the reverse hydrocyclone mentioned is returned to the feed of the primary screen.

8. A method as recited in claim 1, characterized in that the reject from the primary screen mentioned is treated at first in a pressure screen, i.e. a so-called secondary screen, and the reject from the secondary screen is treated in a hydrocyclone treatment so that the lighter accepted fraction from the hydrocyclone treatment is returned to the feed of a previous screening stage.

9. A method as recited in claim 1, characterized in that the reject from the primary screen mentioned is treated at first in a pressure screen, a so-called secondary screen, and the accept from the secondary screen is treated in a hydrocyclone treatment.

10. A method as recited in claim 1, characterized in that the reject from the pressure screen mentioned is treated at least in two treatment steps one of which is a hydrocyclone treatment in which heavy impurity fraction is removed from the reject mentioned, and the other one is a reverse hydrocyclone treatment in which light impurity fraction is removed from the reject mentioned.

11. A method as recited in claim 10, characterized in that the two treatment steps mentioned are preceded by at least one treatment step in a pressure screen.

12. A method as recited in claim 10, characterized in that at least one treatment step with a pressure screen has been provided between the two treatment steps mentioned.

13. A method as recited in any of the preceding claims, characterized in that the heavier fraction accepted by the reverse hydrocyclone treatment is conveyed to the feed of a preceding treatment stage or step.

14. Apparatus for the treatment of fiber suspension in the so-called secondary screening, comprising at least a pressure screen (30), characterized in that the apparatus used for treating the reject from the pressure screen (30) mentioned includes at least one reverse hydrocyclone step (40; 46, 48; 57; 64).

15. Apparatus as claimed in claim 14, characterized in that the apparatus comprises at least one hydrocyclone step (34, 38; 62).

16. Apparatus as claimed in claim 14, characterized in that the apparatus comprises a pressure screen, a so-called secondary screen (32) mounted directly in the reject line (30r) of the primary screen (30).

17. Apparatus as claimed in claim 15, characterized in that the hydrocyclone step (34) mentions is mounted directly in the reject line (32r) of the secondary screen (32).

18. Apparatus as claimed in claim 15, characterized in that the hydrocyclone step (62) mentioned is mounted directly in the reject line (30r) of the primary screen (30).

19. Apparatus as claimed in claim 17, characterized in that the pressure screen (36) is mounted directly in the accept line (34a) of the hydrocyclone step (34).

20. Apparatus as claimed in claim 14, 16, or 19, characterized in that a slotted screen cylinder having a slot width of less than 0.25 mm is used as the screening member in at least one of the pressures screens (30, 32, 36).

21. Apparatus as claimed in claim 20, characterized in that the width of the slot mentioned is less than 0.20 mm.

22. Apparatus as claimed in claim 20, characterized in that the width of the slot in the primary screen (30) is less than 0.20 mm, in the secondary screen (32) less than 0.25 mm and in the screen (36) treating the accept from the hydrocyclone step (34) less than 0.15 mm.