FI103734B - Method and apparatus for treating a fiber suspension - Google Patents

Description

103734

Method and apparatus for treating a fiber suspension

The present invention relates to a method and apparatus for treating fiber suspensions in the paper and pulp industry. In particular, the invention relates to a so-called pulp in the bleaching stage or already bleached. more efficient sorting of waste.

Solutions for similar purposes are known in the prior art in which the pulp at the bleaching stage is led to a vortex cleaning plant, which removes the heavier impurities still present in the pulp, such as sticks, large fiber deposits and the like cylindrical or conical wall. Thus, the coarse, rejectable fraction accumulates on the cyclone surface under the effect of centrifugal force, thereby rapidly consuming sulfur, whereby the cleaning efficiency of said cleaner initially decreases as the material rotation speed drops rapidly due to excess friction. Finally, a hole is created in the wall of the cleaner, from which the material to be cleaned is discharged to the floor of the screen space. In addition, the vortex cleaning plant requires a relatively large space, mainly due to the large number of cyclones. The use of cyclones also has a third disadvantage and is a significant increase in the internal water circulation of the pulp mill and the 20 waste water load. To function effectively, the consistency of the fiber suspension to be cleaned by vortex cleaners, as known, must be one-tenth of a percent, so that the acceptable fiber material is sufficiently precisely separated from the knot particles and sticks.

A further disadvantage of the vortex cleaning plant is the fact that the bleachers most commonly use washers having a pulp consistency of about 1.2%, at least 1.0%. Similarly, a post-bleaching drying machine requires that same feed consistency of about 1.0-12%. However, the vortex cleaning plant will not operate at sufficient consistency at said consistency, but the consistency of the fiber suspension in the plant should be about 0.5%. As a result, after the vortex cleaning plant, the pulp must be precipitated from 0.5% consistency to 1.0-1.2% consistency.

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The disadvantages of the prior art apparatus described above can be eliminated by replacing the vortex purification plant, for example, by a narrow slot screening mode, whereby the task of dozens of vortex cleaners is performed by a single screener. At the same time, the precipitation needed after 5 wheel cleaning plants is also saved. This solution is discussed, for example, in U.S. Patent Nos. 5,470,432 and 5,571,384. The space saving is thus obvious, as is the reduction in liquid load, since the suspension to be treated does not need to be diluted to the level required by the sweeping plant, but can be left at the consistency level of the washer or dryer feeder. This will significantly reduce the need for diluent. Said US patents deal with a solution in which a pressure sorter reject is treated in a sweeping plant, the accept of which is returned to the pressure sorter feed and the reject is removed from the process either completely or at some earlier stage of processing.

A disadvantage or a disadvantage of the aforementioned US patents is that they do not further address what happens to the pressure sorter reject. Ao. the publications merely state that said reject is further processed in a sweeping plant, from which the accept is restored and the reject is removed. In practice, however, it has been noticed that in the solution according to the aforementioned patents, both the accept and the reject of the sweeping plant are exposed to particles which in a way do not belong to them. In other words, it has been found necessary to develop this so-called. the aftermath concept still.

It is known that in conventional sorters using screen drums with holes or slits, the particles are divided into accept, i.e. acceptable fraction, and rejection, i.e. rejecting fraction, mainly based on the particle size. Especially when using a pressure sorter 25, it is known that some compressible particles, such as STYROX crumbs, are squeezed through the openings of the screen plate and are thus accepted. The only way to prevent this in the sorting apparatus is to use a sufficiently small sorting aperture, preferably a narrow aperture, whereby the pressure in the sorter cannot compress the crumbs through the openings. This can be done in the primary screener, but in the same context, some otherwise acceptable fiber material already at this stage will fall into the reject range, which must therefore be sought in the further sorting steps. If a sorter is used for sorting in the secondary stage, these have to use a larger opening size of 3 103734 in order to recover the fibrous material, for example, the above STYROX crumbs will pass through the secondary sorter as such. In other words, it is, for example, almost impossible to separate such type of unwanted material from the acceptable fiber material.

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It is also known that in vortex cleaners, sorting is mainly based on the specific gravity of the particles, whereby in conventional solutions (e.g. sand separation), heavy particles are rejected and the light fraction is recovered. Thus, a traditional vortex cleaner also accepts a lightweight reject, such as STYROX crumbs, as well as other lightweight, e.g., plastic particles, in combination with the fiber.

In order to solve this problem, a so-called. an inverse vortex cleaner disclosed in, for example, U.S. Patent 4,155,839. The purifier in question, and in particular its function, is capable of distinguishing efficiently and accurately between light weight, for example 15 plastic projections, and acceptable fiber material, even though there is no significant difference in the specific gravity of these materials. Such a purifier works by removing the heavier fiber fraction from the tapered tip at its narrower end, in its traditional position at its lower end, and at the central outlet of the lighter reject upper. However, inverse vortex cleaners are also mounted in the opposite direction, i.e. the top of the cone. Further, the inverse vortex cleaner is characterized in that it can be used to precipitate pulp. For example, in the aforementioned U.S. Patent 4,155,639, it was found that with a feed consistency of 1.0 to 1.5%, the acceptance consistency was 1.6 to 2.6%.

One very significant problem that our invention can solve is precisely the one described above in the so-called. to a post-sorting plant, where a prior art solution with narrow slot sorting can prevent e.g. direct contact of plastic particles with the final product. However, since this prior art post-sorting plant includes a vortex cleaner as a downstream device for narrow-gap screening, it accepts lightweight jj, i.e., according to the fiber material. mm. plastic particles Then this lightweight reject remains in the rotating screen until it is ground into small enough pieces to be accepted by the narrow slot sorter. As a result, the lightweight reject gets into the 103734 4 end product and, at worst, causes obvious quality problems there.

In a method and apparatus according to a preferred embodiment of the invention, the so-called. The reject of the 5 primary sorters is led to a pressure screen, the reject of which is further discussed in the so-called.

in the first vortex cleaning stage.

It is still possible to further develop the above method and apparatus so that the accepta of said first vortex cleaner stage is introduced into either a second inverted vortex cleaner stage or a pressure screen.

It is still possible for the above method and apparatus to be developed such that the accept of said inverse vortex cleaner stage or pressure sorter is fed to the secondary sorter input and the reject is removed to the second inverse vortex cleaner stage.

In a method and apparatus according to another preferred embodiment of the invention, the so-called. the reject of the primary screen is taken to a first vortex purifier stage, the accept of which is processed in an inverted vortex purifier, whose accept 20 is returned to the input of the primary screen and the reject is removed from the process or redirected to a suitable further processing point.

The methods and apparatuses of the various embodiments of the invention described above can substantially improve the purity of sorting by utilizing various sorting devices, such as pressure sorter, vortex cleaner and inverse vortex cleaner, in such a way that only their shaving features are utilized. These devices are superbly complementary so that only the particles in the final reject end up in them.

In addition, the apparatus of the invention is substantially more cost-effective to obtain and maintain for a cellulose mill, which, combined with improved efficiency / accuracy of the mill 5103734, results in a state-of-the-art product and method.

Characteristic features of the method and apparatus of the present invention will be apparent from the appended claims.

In the following, the invention will be explained in more detail with reference to the accompanying drawings, in which Figure 1 schematically shows an apparatus of the prior art, Figure 2 schematically shows an apparatus of another prior art, Figure 3 shows an apparatus of a preferred embodiment of the Figure 5 shows an apparatus according to a third preferred embodiment of the invention, and Figure 6 shows an apparatus according to a fourth preferred embodiment of the invention.

According to Figure 1, the prior art arrangement consists essentially of a bleaching tumbler 2, a subsequent vortex cleaning plant 4, a precipitator 6 and a scrubber, most commonly a tumble scrubber 8, and a subsequent bleaching stage 20 tower 10. The bleaching suspension from where it is further fed to the vortex cleaners 4. After the cleaners, the approved material is fed directly to the precipitator 6, which is intended to precipitate the pulp into the feed consistency of the next washer 8. From the precipitator 6, the pulp is fed to the washer 8, from which the washed pulp is removed at a consistency corresponding to the bleaching consistency of the 25 bleaching tom. The reject of the purge cleaners is removed from the equipment for either disposal or recycle to an earlier treatment step.

Figure 2 illustrates another prior art arrangement (US 5,571,384) consisting of a bleaching tumbler 20, a sorting screen 22, a vortex cleaner / purifier, 30 24, a scrubber 26, and bleaching of the next bleaching step, usually the last step (not shown). According to the invention, the bleached pulp in the tower 20 is discharged from the bleaching tubing 20 to the pipeline, either diluted upon removal from the tower or wherein pulp 103734 6 is diluted to a sorting consistency of> 1.0 to 1.2% and fed to a screen 22. equipment to a smaller vortex cleaning plant 24 diluted to a cyclone suitable consistency. Acceptance of the vortex cleaners 24 is returned to the feed of the aforementioned screen 22 or directly to the scrubber 26. The reject, in turn, is either discarded or re-fed to an earlier suspension treatment step.

The apparatus according to an advantageous embodiment of the invention shown in Figure 3 consists of a so-called. a primary sorter 30, the accept of which is derived in a manner known per se from the prior art. ίο main treatment line 30a for further use in the process. The reject or coarse fraction of said primary screen 30, in turn, is led in this embodiment to line 30r to a second stage pressure screen 32 so-called. a secondary sorter whose accept is restored along line 32a to the feeder of the primary sorter 30 and a coarser fraction of reject is led along line 32r to the first vortex purifier stage 34. In some cases, especially when the secondary (represented by a dashed line). The accept, i.e. the lighter fraction, of the first vortex cleaner stage 34 is led along line 34a to the pressure shield 36 and the rejection, or heavier fraction, along line 34r to the second vortex cleaner stage 38. The accept of the pressure sorter 36, i.e. the fraction passed through the screen surface, is led along the line 36a to the feed of the pressure sorter 32 and the reject, or coarser fraction 363, the lighter fraction obtained from the purifier 40 is removed as a reject (line 40r) from the process or passed to a suitable further treatment.

The operation of the apparatus shown in Figure 3 below will be explained in more detail in the light of the examples. In the case of the example, the primary screen 30 has a slot drum having a slot width of 0.15 mm as a screening member. Such a drum accepts, that is, passes only fibrous material suitable for further use. All the larger 7,103,734 fiber particles, sand, sticks and other impurities, such as plastic, get into the reject. The reflux is led to a secondary screen 32, which preferably has a 0.18 mm slit drum. In principle, this drum should also accept only acceptable fiber debris, but for safety, the acceptor of this screen is returned to the feed of the primary screen 5 for re-sorting. The reject of the secondary sorter is led to a first round cleaning stage 34, which, in series, comprises a suitable number of purifiers. The purpose of the first vortex cleaner stage is to separate the reject from the secondary screen into acceptable fiber on the one hand and heavy fraction such as sand on the other. The heavy reject is processed in the second vortex purifier stage 38 to recover and recover the fibrous material useful in the first vortex purifier stage, followed by the removal of virtually pure sand or similar heavy impurities from the reel stage 38. The accept of the first vortex cleaner stage 34 is conducted at a very low consistency (0.4-0.6%) characteristic of the acceptor of the purifiers to a pressure screen 36 with a very small gap, for example 0.12 mm. The purpose is to separate almost all usable fiber fraction from the first vortex purifier stage and return it to the secondary sorter feed. The reject of the pressure sorter 36, which depending on the amount of reject, can be taken either continuously or periodically, is processed in a reverse vortex cleaner 40, the main purpose of which is to separate the lightweight reject from the fibrous material. As an acceptance of the inverse vortex purifier stage 40, the fiber stream taken from the tip of the cone is directed, when thickened, to the feed of the pressure screen 36, through which the fiber fraction is further transported into use. When using a slit drum in different sorters, the slit width of the drum of the secondary screen 32 is less than 0.25 mm, preferably less than 0.20 mm, and the slider 36 is less than 0.20 mm, preferably less than 0.15 mm.

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The apparatus shown in Figure 4 according to another preferred embodiment of the invention consists, as in Figure 3, of a primary screen 30, a secondary screen 32, and a subsequent first and second vortex purifier stages 34 and 38. In fact, the only difference 30 in Fig. 3 is with the heavier fraction removed as an accept being returned along line 46a to the feed of the secondary screen 32 and the lighter fraction removed as a reject 8 103734 is led along line 46r to the second inverted vortex purifier stage 48. Even in this embodiment, it is possible, especially when the secondary sorter is constructed as required, to restore the secondary sorter accept also to the main line 30a after the primary sorter (shown by the dashed line). When a bead drum is used in different sorters, the primary screen 30 has a drum width less than 0.20 mm, preferably about 0.15 mm, and a secondary screen 32 has a drum width less than 0.25 mm, preferably less than 0.20 mm.

The apparatus according to a third preferred embodiment of the invention shown in FIG. the heavier fraction taken is led along line 57a to the feed of the primary screen. In other words, the secondary screen 32 removes the large particles to the vortex purifier stage 34, which sorts them according to their specific gravity either as a reject (line 34r) or to be returned to the feed of the secondary screen 32 by line 34a. The secondary sorter accepts light material, which is led to a reverse sweep cleaning 57, in which the lightweight reject (line 57r) is separated from the process and 2 0 heavier, the fiber fraction is returned to the feed of the primary screen 30. Also in this embodiment, especially when the secondary screen 32 and vortex cleaner assembly 34 are constructed as required, it is also possible to restore the acceptance of the sweep cleaner stage 34 to the main line 30a after the primary screen (shown by dashed line).

The apparatus according to a fourth preferred embodiment of the invention shown in Fig. 6 consists of a primary screen 30, the reject of which is guided according to the prior art to the first vortex purifier stage 62, further reject of the reject 62 along the prior art from a process or a suitable processing stage. A novel solution in the apparatus of this embodiment 30 is to conduct the accept of the first sweep cleaner stage 62 along line 62a to the inverse vortex cleaner stage 64, the heavier fraction being returned along line 64a to the feed of the primary screen 30 and the lighter fraction reject 34 Another novel solution is to flotate the filtrate of the precipitating device 66 following the primary screen in line 30a by flotation to recover the fibrous material.

Preferably, the invention employs a screen cylinder according to F1 patent application 955724, which consists of a set of parallel screen anchors firmly secured to support strands located on the side of the accept space. Preferably, the solution of the present invention utilizes narrow slits having a width of the order <0.25 mm, preferably <0.20 mm, sometimes even close to the order of 0.10 mm. In this context, it is worth mentioning that the shape of the screen element itself can be not only cylindrical, but also conical, 1 ° double conical or even planar.

For example, the rotor according to U.S. Patent No. 5,000,842 may be used as the rotor of said pressure sorter, wherein a so-called rotor is arranged on the surface of the rotor cylinder. pieces to create turbulence that disrupts fiber flocs in its mass. Also, so-called foil rotors 15 may be appropriate in some cases. Similarly, it is possible to use both the outer and inner screen cylinders of the rotor, or even screen cylinders located on both sides of the rotor. Further, in some cases, the use of rotating / rotating screen cylinders may be appropriate.

As seen above, it has been possible to develop a method and apparatus for mass sorting that is very user-friendly and inexpensive in terms of process and energy, as compared to the prior art. It should be noted, however, that "the invention has been described above with reference to only a few embodiments, although in fact the true scope of the invention is much broader and more comprehensive. Thus, the invention is limited only by the appended claims.

Claims (22)

A method of treating a fiber suspension in so-called after-screening where impurities are removed from the pulp using a so-called primary strainer, which impurities are further treated in at least one sieving step, characterized in that the reject from the pressure sieve is treated in one or more treatment steps. which at least one consists of reverse vortex cleaning. 2. A method according to claim 1, characterized in that the reject from the primary strainer 10 is treated first in a pressure strainer, in a so-called secondary strainer, the reject from which is later treated by reverse vortex cleaning. 3. A method according to claim 2, characterized in that the reject from the secondary screen is treated in a vortex cleaning step, the accepted lighter fraction from which is handled in the reverse vortex cleaning step. 4. A method according to claim 3, characterized in that the lighter fraction is first treated in a pressure sieve, the reject from which is led to the reverse vortex cleaning step 20. Process according to claim 3, characterized in that the lighter fraction is treated directly in the reverse vortex cleaning step. 6. A method according to claim 1, characterized in that the reject from the primary screen is led to a vortex cleaning step, the accepted lighter fraction from which is led to reverse vortex cleaning. Process according to claim 6, characterized in that the accepted heavier fraction from the reverse vortex cleaning step is fed back to the primary screen input. 30 8. A method according to claim 1, characterized in that the reject from the primary strain is processed first in a pressure strainer, in a so-called secondary strainer, the reject from which is treated by swirl cleaning, so that the easier acceptance function from the swirl cleaning is returned to the input of some of the preceding strain. * 14 103734 9. A method according to claim 1, characterized in that the reject from the primary strainer is first processed in a pressure strainer, in a so-called secondary strainer, the acceptance from which is treated by reverse vortex cleaning. 5 10. A method according to claim 1, characterized in that the rejection of the pressurized screen is processed at least in two treatment steps, one of which is a vortex cleaning step, in which the heavy window cleaning fraction is removed from the rejection and the other is an inverse vortex cleaning step in which the light contamination fraction is removed. . 10 11. A method as claimed in claim 10, characterized in that the two treatment steps are preceded by at least one treatment step with the pressure screen. Method according to claim 10, characterized in that between the two treatment steps there is provided at least one treatment step with the pressure screen. Method according to any one of the preceding claims, characterized in that the accepted heavier fraction from the reverse vortex cleaning step is guided to the input of some of the preceding treatment steps or stages. 20 Apparatus for treating a fiber suspension in so-called after-screening, which means comprises at least one pressure sieve (30), characterized in that the means for treating the reject from the pressure sieve (30) include at least one reverse vortex cleaning step (40; 46,48; 57, 64). 25 Devices according to claim 14, characterized in that the devices include at least one swirl cleaning step (34,38; 62). Devices according to Claim 14, characterized in that the devices include a pressure strainer (30r) directly connected to the reject line (30r) of the primary strainer (30), a so-called secondary strainer (32). Devices according to claim 15, characterized in that the swirl cleaning step (34) is directly connected to the reject line (32r) of the secondary screen (32). 103734 Apparatus according to claim 15, characterized in that the swirl cleaning step (62) is directly coupled to the straight line (30r) of the primary screen (30). Devices according to claim 17, characterized in that the pressure screen (36) is directly connected to the accept line (34a) of the vortex cleaning step (34). Devices according to Claim 14, 16 or 19, characterized in that in the at least one of the pressure screens (30, 32, 36) a screen drum is used as a screen, the slit width of less than 0.25 mm is used. Devices according to claim 20, characterized in that the slot width is less than 0.20 mm. Devices according to Claim 20, characterized in that in the primary screen (30) the slot width is below 0.20 mm, in the secondary screen (32) below 0.25 mm and in the screen (36) which processes the acceptance from the wire rope step (34) below 0 , 15 mm.