METHOD AND APPARATUS FOR TREATING FIBER SUSPENSION

Abstract: The present invention relates to a method and apparatus for treating fiber suspension, for example, sorting or grading. The method and apparatus in accordance with the invention is especially suitable for the treatment of fiber suspensions in the pulp and paper industry. The most serious defect in the known apparatuses is that the relative amount of the reject, in other words the material which is unqualified for the finished product, adjacent to the screen surface rises directly in proportion to the distance from the initial or front end of the screen surface since acceptable fraction is discharged through the screen surface, but the reject is discharged only after it has traversed the whole length of the screen surface. In the arrangement in accordance with the present invention the treatment space for the suspension is divided into several zones (21, 22, 23), from each of which the reject accumulated in them is discharged to its own treatment zone (24). New unsorted suspension is also fed in said zones so that, for example, in a screen apparatus of a conventional size there may be three separate zones treating unsorted pulp and one zone treating reject.

Description: The present invention relates to a method and apparatus for treating fiber suspension, in which method fiber suspension is fed to the treatment apparatus, and impurities and too large particles are separated from the suspension. The apparatus according to the invention comprises a screen apparatus communicating with a new type of a rotor. The method and the apparatus in accordance with the invention are especially suitable for screening suspensions in the sorters of the pulp and paper industry. The sorters known from the prior art most often comprise an apparatus to which pulp is fed continuously and which has one or more stationary screen cylinders and a rotor moving adjacent to the surface of these cylinders. Pulp is fed axially to the space between the screen surfaces/surface and the rotor, subsequent to which and due to the operation of the rotor, the pulp is caused to follow a circulating, spiral movement. In the screening the acceptable fraction passes through the screen surface and is discharged as an accept and the coarser fraction flows in the axial direction along the screen surface from the inlet end to the outlet end of the cylinder, from which end it is discharged as a reject. Fig. 1 is a schematic illustration of the above described prior art sorter, which, for simplicity, has only one screen cylinder and a closed rotor inside the cylinder. Fig. 2 is a graphic illustration of how by means of the described method the amount of the reject is increased between the screen surface and the rotor evenly as a function of the height of the cylinder. Fig. 3 is a corresponding graphic illustration of the amount of the accept flowing through the screen cylinder as a function of the height of the screen cylinder. It can be seen that the accept flow is at its maximum, of course, at the upper end of the screen cylinder, and therefrom the amount of accept decreases rapidly, because the relative proportion of the reject in the material to be screened increases rapidly and, on the other hand, the reject tends to clog the screen surface. Thus the problem is that the actual capacity of the screen surface is only partially in use, in other words the screen surface is used only at partial efficiency. A possible solution to the above described problem could be, for example, the use of a conic rotor, whereby more material to be screened may be introduced between the screen cylinder and the conic rotor. Also in the case it may be maintained that the relative proportion of the reject in the space between the screen surface and the rotor increases in the similar way as in the previously described embodiment. However, it has to be admitted that a slightly better screening capacity of the screen surface is achieved by a conic rotor than by a rotor in accordance with Fig. 1, because fresh unsorted suspension may be fed deeper in the space between the screen surface and the rotor. A second possible solution is described in US Patent Publication 4642189 which discloses a substantially conic rotor, the upper end of which is, however, formed in such a way that the pulp to be fed is divided in a way into three annular portions one within the other, and each of which portions is fed in at different height into the space between the rotor and the screen surface. Figs. 4 and 5 are graphic illustrations of what happens in reality to the amount of the reject and to the accept flow passing through the screen surface in the function of the height of the screen cylinder. It is to be seen in Fig. 4 that the amount of the reject increases gradually at the same place with the feed of the material. A broken line in the figure shows the situation where the feed between the rotor and the screen surface is carried out in one stage. Thus the slashed area shows the advantage achieved with the arrangement in accordance with said publication compared with the arrangements known in the prior art. Fig. 5 clearly illustrates the three-stage-nature of the feed of the material. Each feed stage generates an addition or in other words "a step" in the accept flow, and after each feed the accept flow reduces according to Fig. 3, yet in such a way that the speed at which the accept flow reduces after each feed, increases the deeper the feed between the rotor and the screen surface is carried out, because the amount of the reject decelerates the accept flow the more intensively the greater the amount of the reject there is in the space. The broken line in the figure signifies the situation in which the feed is carried out in one single stage directly from the end of the screen cylinder. The slashed area shows the amount of the volume flow which passes through the screen surface thanks to this method of feeding. In said US publication the feed stages of the unsorted pulp take place in the steps of about 20 % considering the total height of the screen cylinder. It is not advantageous to arrange feed stages below the vertical middle of the screen cylinder, because they would hardly be of any use anymore compared with the above described curves. Since it has been necessary to arrange a feed of dilution liquid through the rotor to the lower part of the screen cylinder, the problem with the arrangement in accordance with said publication seems to be the excessive thickening in the space between the rotor and the screen surface. A common problem to all described arrangements of the prior art is the even increase of the amount of the reject between the screen surface and the rotor. Because the reject generates a particular flow resistance to the screening process, the screening efficiency achieved is not much better than that of a traditional screen apparatus illustrated in Fig. 1, unless it is possible to remove reject from the area between the ends of the screen apparatus, in other words to reduce the flow resistance caused by the reject in the space between the rotor and the screen surface. By using the method and apparatus in accordance with the invention it is possible to eliminate or minimize the defects of the apparatuses according to the prior art and thus to increase the capacity of the screen apparatuses significantly without a need for changes in the actual screen cylinder or for extra costs the improvements in accordance with the invention might bring to the users. According to the present invention there is provided a method of treating fiber suspension in which method fiber suspension is fed into a screening apparatus having a plurality of screening zones, and in which the suspension is divided into two fractions in the apparatus, so called accept and reject, which are discharged from the apparatus as separate flows, characterized in that the reject is guided from at least one zone so as to by-pass at least one of the following screening zones to a subsequent screening zone, from which at least a portion of the acceptable fraction still in the suspension is discharged. Also according to the present invention there is provided an apparatus for treating fiber suspension which has a plurality of screening treatment zones and which comprises an outer casing, a conduit for the pulp to be fed into the outer casing, a conduit for the acceptable fraction, the so called accept, a conduit for the fraction being discharged, the so called reject, at least one screen cylinder arranged inside the outer casing and a rotor member, movable relative to the surface of the screen cylinder, characterized in that the rotor includes ducting members, by means of which ducting members the reject material accumulating in the so called treatment space is led from a separate initial screening treatment zone of the treatment space between the rotor and the screen surface past an adjacent screening treatment zone so as to by-pass such and to a subsequent screening treatment zone. The invention is described below in detail, by way of example, with reference to the accompanying drawings, in which: Figs. 1 - 5 are schematic illustrations of apparatuses and their capacities as described in the prior art; Fig. 6 is a schematic illustration of the operating principle of the method and apparatus in accordance with the present invention; Fig. 7 is a graphic illustration of the resistance caused by the reject of a screen apparatus operating on the principle in accordance with Fig. 6; Fig. 8 is a graphic illustration of the accept flow through the screen surface of the screen apparatus operating on the principle in accordance with Fig. 6; Fig. 9 is a schematic illustration of an alternative operating principle of the method and apparatus in accordance with the invention; Figs. 10 and 11 are graphic illustrations of the amount of reject and the accept flow of the alternative embodiment of Fig. 9 in the same way as Figs. 7 and 8 illustrate the alternative of Fig. 6; Fig. 12 is a graphic illustration of the comparison between the method and apparatus according to the invention and those of the prior art; Fig. 13 is a schematic illustration of an embodiment of an apparatus in accordance with the invention; and Fig. 14 is a schematic illustration of another embodiment of the apparatus in accordance with the invention. The invention relates in principle to a modification of the screen apparatus in accordance with Fig. 1, which mainly comprises an outer casing 1 having a conduit 2 leading thereinto for the feed of pulp, a conduit 3 for the outflow of the accept and a conduit 4 for the outflow of the reject. Inside the outer casing 1 there is a screen or filter surface 5, adjacent to which surface is located a member or so called rotor 6 movable relative to the surface 5. Fig. 6 is a fragmentary schematic development of the treatment space of the apparatus of Fig. 6 and schematically illustrates the operating principle of the method in accordance with the invention. In the case of Fig. 6 the treatment space 10 of the fiber suspension, e.g. the space between screen surface 5 and rotor 6 of Fig. 1, is divided axially in three zones 11, 12 and 13, which can be of the same size in the axial direction, but they may, alternatively, differ in size. Pulp is fed conventionally substantially in the axial direction to the treatment space 10, but a portion of the untreated pulp is exceptionally guided past the first zone 11 and directly to the second zone 12 by means of one or more ducting members 14, whereby the screen surface of both the first zone 11 and the second zone 12 receives unscreened pulp to be treated. After the screening process is over in the first zone 11, in other words once the pulp has been circulated by executing its spiral passage downwardly along the screen surface and the accept has flowed through the openings of the screen surfaces and the reject is separated to its own layer, at least part of the reject by-passes the next zone by being led by ducting members 15 directly past the second zone 12 to the third zone 13, in which the separation of the acceptable fraction still flowing with the reject is carried out and the acceptable fraction is guided to the accept. Thus the amount of the reject flowing to the second zone 12 and substantially decelerating the separation is decisevely reduced, whereby the improved capacity of the screen apparatus shown in Figs. 7 and 8 is achieved. Members 17 may be arranged either in some cases for by-passing zone 13, i.e. leading the reject from zone 12 to the rejects compartment or only for cleaning the screen surface in zone 13, whereby the member 17 is either hollow or solid, e.g. conventionally foil-like. Fig. 7 represents the amount of the reject accumulating in the treatment space of the screen apparatus for realizing the method in accordance with the invention as a function of the height of the screen surface exactly the same way as Figs. 2 and 4 shown the amount of the reject in the prior art embodiments. At the beginning A of the first zone 11 the amount of the reject is, of course, 0, from which it increases evenly to the value r. Thereby the reject is guided directly past the second zone 12 to the third zone 13. Thus the amount of the reject at the beginning B of the second stage decreases back to zero, because it receives untreated suspension, from which value it rises again to the value r when reaching the border surface C between the second and third zone. At the beginning of the third zone 13 the amount of the reject rises to the value 2r, because the rejects of the first and second zone have joined together. In the third zone 13 the increase in the amount of the reject corresponds in the embodiment of the figure 1 to the lowest third of the increase in the amount of the reject of an apparatus in accordance with the apparatus of Fig. 1 illustrated in Fig. 2. Fig. 7 shows by broken lines the situation of Fig. 2, whereby the slashed area presents the benefit achieved by the present invention compared with the apparatuses known in the prior art. Fig. 8 illustrates as a function of the height of the screen surface the change in the amount of the accept in the embodiment according to the invention. It has to be noted at this point that the uppermost part a is of similar size in Figs. 3, 5, and 8, because it illustrates the maximum flow through a particular screen surface at a particular pressure difference. Thus it does not make any difference how great is the amount of the suspension fed to the treatment space, in other words how open the treatment space is in the upper end of the screen cylinder (whether it is a conic or cylindrical rotor). Only the pressure difference prevailing over the screen surface is relevant. As is seen in the figure, the volume of flow through the screen surface decreases during the first stage exactly corresponding to Fig. 2 to the value a₁, until the border B of the first and second zone is reached, whereby the volume flow rises back to its maximum value a, because there is no reject in the second zone decelerating the screening. The curve of the second zone is exactly parallel to that of the first zone, in other words the volume of flow reduces to the value a₁. When the second stage changes to the third at point C the volume flow reduces to the value a₂, which again is in accordance with the Fig. 2, as the broken line in Fig. 8 shows. The slashed area shows the greatest amount of the volume flow achieved by the method and apparatus in accordance with the invention compared with the arrangements known in the prior art. Fig. 9 is a fragmentary schematic development of part of the treatment space of an alternative embodiment of the method and apparatus in accordance with the invention, in which the treatment space is divided into four separate zones 21, 22, 23 and 24. The pulp flowing into treatment space 10 is divided into three portions in such a way that about one third of the untreated or unsorted suspension is brought directly to the first zone 21, about one third is guided by a ducting member 25 to the second zone 22, and about one third through guiding member 26 to the third zone. Respectively the reject generated in the first zone is guided past the second and third zones directly to the fourth zone 24 by a ducting member 27 and the the reject of the second zone is similarly guided past the third zone by a ducting member 28. The reject of the third zone flows directly to the fourth zone 24. Thus the reject of the previous zones does not flow to the second 22 or third 23 zone as would decelerate the screening. Member 29 may be arranged either in some cases for by-passing zone 24, i.e. leading the reject from zone 23 to the rejects compartment or only for cleaning the screen surface on zone 24, whereby the member 29 is either hollow or solid, e.g. conventionally foil-like. Figs. 10 and 11 correspond to Figs. 7 and 8 and are graphs representing reject accumulation and change in amount of accept of the apparatus of Fig. 9. In Fig. 10, the amount of the reject is zero of course at the beginning K of the first zone but also at the beginning L of the second zone and M of the third zone and increases in each of the first three zones from zero to the value r₁. At the beginning N of the fourth zone the amount of the reject rapidly increases to the value 3r₁, because the rejects of the first and second zones, in other words the rejects of all preceeding zones are also guided to the said zone. The slashed or hatched area shows the advantage gained by the method of the invention compared with the conventional system. Fig. 11 shows that both at the beginning of the second and the third zone the pulp flow of the accept increases to its maximum a decreasing at the end of each zone to the value a₄ due to the reject accumulated in that zone. At the beginning of the fourth zone the volume flow decreases to the value a₅, because the amount of the reject in the material to be screened has increased for the reason already described above. Similarly to the previous drawings the slashed area illustrates the advantage gained by the method and apparatus in accordance with the invention compared with the apparatuses of the prior art. Fig. 12 illustrates the comparison of the graphs of Figs. 3 and 5 showing the operation of the apparatuses of the prior art and the capacity being achieved by the method and apparatus in accordance with the invention. The unbroken line illustrates Fig. 3 showing the conventional technique, the broken line the operation of the arrangement in accordance with the US Patent Publication 4642189 (Fig. 5) and the dotted line the capacity of the method and apparatus in accordance with our present invention. It has to be noted that in illustrating our invention in Fig. 12 an arrangement with four zones has been used, in which the division of the zones is similar to that of the apparatus in accordance with said US patent. This arrangement is made in order to make the comparison between the capacities of the different arrangements in the best possible way. It can be seen in the figure that in the first zone there are no differences to be seen between the apparatuses and all devices are capable of treating the same amount of stock in the first zone the accept being the same in every case. In the second zone by the improvement (horizontal lining+slashing) brought about by the present arrangement compared to that of US patent (slashing) is double and in the third zone already is at least triple. Notwithstanding the fact that our invention already in this kind of comparison is superior to the known arrangements, it has to be noted that the difference compared with the known technique could be increased considerably if the zone division in our invention were, for example, similar to Fig. 9. Thereby the advantage to be achieved compared with the apparatus in accordance with the US patent could be of quite another scale. Fig. 13 discloses a schematic horizontal cross-sectional view of a screen apparatus in accordance with the invention in which the treatment space 10 is formed between two stationary screen cylinders 18 and 19. The arrow shows the direction of the rotation of the rotor comprising foils or ducting members 14 and 15. A rotatable rotor is arranged into the treatment space 10, which rotor comprises a plurality of axial or almost (± 30°) axial hollow foils 14 and 15. Foils 14 as well as the foils 15 have been arranged by turns adjacent to the screen surfaces 18 and 19 and their cross-section is formed so as to keep the screen surfaces clean. The length of the foils 14 and 15 as ducting members may vary in the same way as in Figs. 6 and 9, or, of course, it there are more zones even the lengths of the foils may vary respective to the height of the screen surface. In Fig. 6 foils 14 and 15 are mounted to plates 16 to be better seen, to the lower one of which the foils 17 are mounted to keep the screen surface of the third zone 13 clean. The intermediate plates 16 have openings for the foils 14 and 15, through which openings the unserted suspension is brought from the inside of foil 14 to the second zone and from which the reject of the first zone 11 flows to the foil 15 which guides the reject past the second zone 12 to the third zone 13. When it is necessary the intermediate plate 16 may otherwise be completely closed, whereby no suspension which has flown to the first zone 11 or reject separated from the suspension may flow to the second zone. Corresponding intermediate plates are used, of course, also between other zones. If the apparatus is a three-zone type as in Fig. 6, it is advantageous to arrange the relation of the sum of the cross-sectional surfaces of foils 14 and the cross-sectional surface of the whole treatment space to be about 1:2, whereby one half of the suspension flows to the first zone 11 and the other half through the hollow foils 14 to the second zone 12. It is clear that both the hollow foils receiving unsorted suspension and reject and more precisely the ends of the foils may be formed e.g. scoop-like, so as to better let in the pulp. Although the references made in the above description are mainly made to refer to the embodiment in accordance with Fig. 6, the same features are true also for the embodiment in accordance with Fig. 9, in which the only exception is, for example, the extension of a ducting member 26 or a ducting member 27 over two zones, in other words said foils forming ducting members pierce the intermediate wall between different zones. Respectively, it is advantageous to divide in this embodiment the unsorted suspension flowing into the treatment apparatus so that the sum of the cross-sections of the foils 25 and 26 is two thirds of the treatment face, whereby the pulp is distributed evenly to the different zones. Further, similar to Fig. 6 conventional foils 29 might be arranged to the zone 24, with which the screen surface is kept clean. It should be emphasized that both in Fig. 6 and Fig. 9 the foils 17 and 29 in the treatment zones 13 and 24 of the reject are closed. Fig. 14 discloses as another alternative embodiment of the invention a screen apparatus in which similar to the previous embodiment, there are two screen cylinders 30 and 31, a treatment space 10 between them and a rotor rotating in it. The arrow indicates the direction of rotation of the rotor. The rotor comprises, however, two undulating or wavily bent or curved plates 33 and 34 or like, which are arranged at certain distances from each other so that cross-sectional area of the space 32 between the plates 33 and 34 and the screen surfaces is about the same as the open surface between said plates. Thereby the construction corresponds in the operational principle the arrangement shown in Fig. 6. If, again, operation similar to that of Fig. 9 is required, then, for example, the distance of the plates from each other has to be increased so that the cross-sectional area between them is two thirds of the cross-sectional area of the treatment space, whereby also two thirds of the suspension is drawn to the said space. The division between the second and the third treatment zones may be carried out later. The second alternative is to arrange plate surfaces one within the other exactly as many as there are treatment zones of the unsorted suspension, whereby the suspension may be divided into different zones preliminarily already when flowing into the treatment space. By arranging the bendings of the plates 33 and 34 in suitable form, it is possible to ensure the effect of cleaning the screen surface of the rotor in accordance with the invention. Plates 33 and 34 are mounted by their lower end on a plate corresponding the intermediate plate 16 of Fig. 6, and on the other side of which plate, plates 35 and 36 are mounted, and the open surface 37 which is between the plates 35 and 36 is arranged to meander crossing the open surface of 32 between plates 33 and 34 in the way that the reject accumulating in the parts remaining between the plates 33 and 34 and screen surfaces 30 and 31 is easily drawn through the openings of the intermediate plate to the open space between plates 35 and 36. Respectively, also by varying these plate arrangements it is possible to create a screen apparatus with four or more zones as is already briefly shown above. Although only two different constructional alternatives have been shown above for the rotor of the apparatus in accordance with the invention and primarily two different zone divisions, it is clear that the scope of invention extends also to other rotor alternatives realizing the same principal requirements and that the number of the zones is restricted only by the production technique of the rotor and the height of the screen cylinder in use. It can clearly be seen from the above examples that the more feed zones for the suspension and discharge zones for the reject there are in the apparatus, in other words the more often the reject is discharged from the treatment space during the screening process the greater benefit may be achieved by the method in accordance with the invention. It is not crucial importance for the method and apparatus in accordance with our invention, whether there are one or more screen cylinders in the apparatus to which our invention is applied. Although both previous examples have two screen cylinders, the present invention may operate quite effectively with one screen cylinder. Thereby the rotor may be formed in another way, which arrangement is also included in the scope of protection of our invention. Thus the disclosed examples are not intended to restrict our invention in any way apart from what is defined in the accompanying claims which define the scope of our invention and its extent. Only a few most advantageous embodiments have been illustrated above, the method and apparatus according to which may be applied in conjunction with the already existing technique. Thus, for example, the rotors in accordance with our invention may directly be installed in existing prior art machines to replace the the rotors of already existing screen apparatuses, whereby the capacity of said screen apparatuses may be increased without any external changes in the apparatus.

Claims: A method of treating fiber suspension in which method fiber suspension is fed into a screening apparatus having a plurality of screening zones, and in which the suspension is divided into two fractions in the apparatus, so called accept and reject, which are discharged from the apparatus as separate flows, characterized in that the reject is guided from at least one zone so as to by-pass at least one of the following screening zones to a subsequent screening zone, from which at least a portion of the acceptable fraction still in the suspension is discharged. A method according to claim 1, characterized in that the untreated fiber suspension is divided into as many portions as the number of initial suspension treatment zones as provided and the portions are fed as a continuous flow to their respective initial treatment zones, from which reject is discharged respectively to a subsequent screening treatment zone by guiding it from each initial treatment zone of the suspension past the next treatment zone or zones of the suspension directly to a subsequent screening treatment zone and letting the reject flow from the initial treatment zone adjacent to the reject treatment zone directly to the reject treatment zone. An apparatus for treating fiber suspension which has a plurality of screening treatment zones and which comprises an outer casing (1), a conduit (2) for the pulp to be fed into the outer casing, a conduit (3) for the acceptable fraction, the so called accept, a conduit (4) for the fraction being discharged, the so called reject, at least one screen cylinder (5) arranged inside the outer casing (1) and a rotor member (6), movable relative to the surface of the screen cylinder (5), characterized in that the rotor includes ducting members (14, 15, 16; 25, 26, 27, 28; 33, 34, 35, 36), by means of which ducting members the reject material accumulating in the so called treatment space (10) is led from a separate initial screening treatment zone (11, 21, 22) of the treatment space (10) between the rotor and the screen surface past an adjacent screening treatment zone so as to by-pass such and to a subsequent screening treatment zone (13; 24). An apparatus according to claim 3, characterized in that the ducting members (14, 15, 25, 26, 27, 28) of the rotor are hollow foil-like members, which at the same time act to keep with their outer surface the screen surface clean and through their hollow inner space guide reject from different zones (11, 21, 22) to its own treatment zone (13, 24). An apparatus according to claim 4, characterized in that the ducting members (14, 15, 25, 26, 27, 28) are mounted to intermediate plates (16), which have openings at the members (14, 15, 25, 26, 27, 28) so that the reject may flow from the openings into the members (14, 15, 25, 26, 27, 28) or respectively out from it or that members (26, 27) are able to be arranged through the openings An apparatus according to claim 3, characterized in that the members (33, 34, 35, 36) of the rotor are undulating or bent unround apparatuses, which are plate-like or like of their construction leaving a space (32, 37) therebetween, through which suspension or reject is guided to different zones of the treatment space or from different zones to the treatment zone of the reject.

Inventors: LJOKKOI, RISTO
Applicants: A. AHLSTROM CORPORATION
Application year: 1988
Main classification: D21D5
IPC codes: B01D29, D21D5