Method and pressure screen for screening fibre suspension

(54) (56) (57)

PATENT INVENTOR INVENTOR'S OFFICE NAME

QUOTES PUBLISHED: SUMMARY:

00130

Metso Paper Inc, Fabianinkatu 9 A

Jouko Hautala, Tampere FI AWAPATENT AB

Process and strainer for sieving fiber suspension

Helsinki Fl

A method and a strainer for sieving fiber suspension. In the process, fiber suspension is passed through two successive screening steps (A, B) arranged in the same strainer so that acceptance from the first screening step (A) is directed to the second screening step (B) and acceptance from the second screening step (B) is removed from the pressure screen. Rejects from the second screening step (B) are returned to the input side of the first screening step (A) for etching. The pressure screen has a first screen cylinder (9) with an inlet space (VI) into which fiber suspension is introduced. A second screen cylinder (10) is arranged at a distance from the first screen cylinder. Acceptances passed through the first screen cylinder (9) are led into the inlet space (V5) of the second screen cylinder through a channel inside the rotor (3) by means of pumping blades (5) fixed in the rotor. The inlet space (V5) of the second screen cylinder (10) is arranged axially after the inlet space (VI) of the first screen cylinder (9).

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The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code.

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The invention relates to a method for sieving fiber suspension in separate screening steps arranged in a single screening screen, wherein acceptance from a first screening step is led to a second screening step, acceptance from the second screening step is removed from the printing screen, rejections from the second screening step are returned to the input side of the first screening step. for re-sieving and rejecting, only the first sieving step is removed from the sieve.

The invention further relates to a pressure suspension screen for fiber suspension, which screen has a housing, a rotor mounted to rotate inside the housing, a first inlet space formed inside the housing and on one side bounded by a first screen cylinder, a first intermediate space formed on the second side of the first the screen cylinder, a second inlet space on a side bounded by a second screen cylinder mounted coaxially with the first screen cylinder, an outlet space formed outside the second screen cylinder and an input channel for introducing fiber suspension to be screened into the screen, the first intermediate space communicating with the second inlet space for passing the acceptance passed through the first screen cylinder to the second screen cylinder; the inlet space is connected to a reject outlet channel and the outlet space is connected to an acceptance outlet channel and foil blades are coupled to the rotor so that they sweep the surface of the cylinders on the respective inlet space.

Fiber suspension is treated by being screened before being inserted into a paper machine or the like, to cause various contaminants, sticks and other particles which degrade the quality of the web to be made;

515 574 shall be removable. This is normally done by using a screen cylinder provided with perforations in such shape and size that an accepted fraction, i.e. acceptance, is able to pass through the perforations as easily as possible, while too large fibers and impurities cannot pass through. The perforations in the screen cylinder may be round or elongated holes or parallel slots formed in the surface. In order for the sieve's capacity to remain sufficiently high, the perforations must in practice be greater than what is actually required for the acceptable fiber size. As a result, knitters, fiber bundles and other contaminants always penetrate the screen to some extent, which requires screening in several steps to achieve sufficient purity. Similarly, any acceptance is always effected with the reject regardless of the size of the perforations, and the reject must then be re-screened for recovery and recirculation of the acceptance.

In order to make the screening process easier and more efficient, we have tried to create different filters or filters, where the screening takes place in a number of different steps. Such devices are described e.g. in Fl Patent Application 309/67 and Fl Patent Specifications 47789 and 51221.

Fl Patent Application 309/67 discloses an apparatus having two screening units functionally coupled to one another. In the apparatus, suspension to be screened in the first screen is introduced into whose outlet channel acceptance is guided. Discarded fraction, i.e. reject, is then diluted with water and led to the second screening step, from which acceptance is again led into an outlet channel, while reject is discharged through another channel. This solution requires water to be added to the fiber suspension, which increases the amount of water to be treated later and requires additional measures to ensure that the fiber suspension to be passed through the inlet box has a suitable density.

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Fl-laying publication 47789, for its part, describes an apparatus which also exhibits two serially connected screening steps. However, the functional interconnection of these steps differs from the one described above. In this solution, the fiber suspension to be screened is introduced in the first screening step and reject is removed therefrom. Acceptance is then led to the second process step, from which the reject is again removed and acceptance is forwarded to the next process step. In this solution too much acceptance is thrown out with the reject, which requires a new separate reject screening step.

FI Layout Specification 51221 describes a solution in which a second screening surface is provided between the reject channel and the suspension to be screened. Liquid and some fibers are able to return into the suspension to be screened through the second screening surface. In practice, this solution corresponds to a solution in which the reject is led to a second separate screening step and the acceptance obtained from the second step is recirculated and mixed with new fiber suspension to be introduced in the first screening step. However, the operation of the apparatus is very unreliable, since in order to conduct fluid and fibers from the reject channel into the fiber suspension insertion space, there is a pressure difference between them, which is in practice contrary to the pressure structure of the apparatus. Thus, no proper secondary screening takes place, although the apparatus utilizes impulse-producing blades for drying the surface of the reject screen, and hardly any real advantage is achieved.

The object of the present invention is to provide a method and a pressure sieve for sieving fiber suspension, which avoids the aforementioned difficulties and provides effective multi-stage sieving in a single sieve. The method according to the invention is characterized by the acceptance of the first screening step

5T5 574 to the second screening step through a channel formed inside a rotor of the screen.

The pressure screen according to the invention is characterized in that a pumping space is formed inside the rotor to extend through it between the first intermediate space and the second inlet space, the first intermediate space and the second inlet space communicating with each other through said pumping space, to the pumping blade. for rotating with it are arranged between the pumping space and the second inlet space, which pumping blades pumps acceptance obtained from the first screening step from the pumping space to the second inlet space.

An essential feature of the invention is that fiber suspension is screened in two successive screening steps so that rejections from the second screening step are returned directly to the feed side of the first screening step through the rotor of the screen. An essential feature of the apparatus is that acceptance from the first screening stage is led through a space formed in the center of the rotor to the input space of the second screening stage and the rotor further exhibits a set of pumping blades which provide a suspension flow through the screening stage.

An advantage of the invention is that fiber suspension can be effectively screened with a single multistage strainer, which forms a uniform whole, while the number, piping and valves required for screening decreases, less pumping is required and energy consumption decreases. A further advantage of the invention is that the apparatus requires less electrification and instrumentation and less space, which leads to a reduction in investment costs.

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In the following, the invention will be described in greater detail with reference to the accompanying drawings, in which Figure 1 is a schematic view of an embodiment of the pressure screen according to the invention, Figure 2 schematically illustrates the function of an application of the pressure screen shown in Figure 1, Figure 3 schematically illustrates the function of a second application of the pressure screen shown in Figure 1, Figure 4 is a schematic view of another embodiment of the pressure screen according to the invention, Figure 5 schematically illustrates the operation of the pressure screen shown in Figure 4, Figure 6 is a schematic view of a further embodiment of the pressure screen according to the invention, Figure 7 is a schematic view of yet another embodiment of the pressure screen according to the invention and Figure 8 schematically illustrates the function of the screen shown. in Figure 7.

Figure 1 shows schematically a structural embodiment of the pressure screen according to the invention. In the other figures, the same reference numerals as in Figure 1 are used for the corresponding parts, so they should not be explained again below. The apparatus has functionally corresponding parts and components, which in the description of the figures are specified with the order numbers first, second, third, etc., regardless of the way in which these parts or components are connected to and connected to the functional process of the apparatus. As used in the claims, the definitions provided with an order number first, second and third should be interpreted as defined in the specification, instead of considering them as terms describing the order in the fiber suspension flow process and its screening.

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The pressure screen has a body comprising a housing 1 and an upper part consisting of a lid 2. Inside the body is mounted a rotor 3 for rotating in a manner known per se, e.g. by using a motor (not shown in the figures) and V-shaped belts (not shown) arranged between the motor and a pulley 3a. At one end of the rotor 3, ie. at its upper end in the figure, a capsule-like rotor frame 4 is provided. The frame 4 is connected to the rotor 3 by means of arms 3b and it has a cylindrical or conical shape and is hollow in the middle. Further, pumping means 5 acting as a pumping means are connected to one end of the rotor frame 4, and a cover plate 5 is provided above the blades. Foil blades 6 and 7 are further connected to the rotor frame 4. Fiber suspension is introduced through an inlet channel 8 which leads to a first inlet space VI, which belongs to the first screening step in the screen. The inner surface of the inlet space VI is defined by a first screen cylinder 9, which is arranged at the first end of the screen and through whose perforations an accepted fraction of the suspension, called acceptance, flows into a first gap V2 inside the screen cylinder 9. From the gap V2 the acceptance flows between the root shaft and its capsule-like frame 4 forward, i.e. upwards in the figure, into a pump space V3. As the rotor 3 rotates, the blades 5 force the fiber suspension into a second gap V4. From the second intermediate space V4 arranged in the direction of the second end of the screen from the first screen cylinder 9, ie. in the upper part of the screen of the figure, the acceptance flows further into a second inlet space V5, which belongs to the second screening stage. The inner surface of the second inlet compartment is defined by the capsule-like frame 4 of the rotor and its outer surface is delimited by a second screen cylinder 10 arranged in the axial direction at a distance from the first end of the screen towards the second end of the screen. In the second screening step

515 574, the acceptance flows through the perforations of the second screen cylinder 10 into an outlet space V6 formed between the housing 1 and the second screen cylinder 10. From the outlet space V6, the acceptance is removed through an outlet channel 11. From the second inlet space V5, the reject is led. the suspension discarded from the second screening step by a thrust action produced by the pump blades 5, i.e. downwardly in the figure so that it returns into the first inlet space VI and thus is again screened through the first screen cylinder 9. The remaining reject is removed into a reject space V7 which is separated from the inlet space VI by an annular or conical plate 14. further through a channel

12th To the second intermediate space V4 is further connected a duct 13 through which air generated in the screen may be removed. Channel 13 can also be used to remove so-called light reject from the screen. As the rotor 3 rotates, the foil blades 6 and 7 connected to the rotor screen 4 also rotate and thus sweep the surface of the screen cylinders 9 and 10 on their feed side, so that the rejects and fibers collected on the surface are released and return to the fiber suspension in a known manner. The screen shown in the figure further requires a shaft seal 23 which seals the rotor shaft so that the fiber suspension and liquid contained in the fiber suspension do not flow to the storage of the rotor shaft. In addition, between the upper edge of the screen cylinder 9 and the frame 4 of the rotor, a seal 20 should be provided to prevent flow of the fiber suspension present in the inlet space VI between the screen cylinder 9 and the rotor frame 4.

Figure 2 shows a functional diagram of the pressure screen shown in Figure 1. Fiber suspension to be screened is introduced into the pressure screen through channel 8. In Figure 2 and in Figures 3, 5 and 8, to be discussed below, the screen cylinders of Figures 1, 4 are shown. 6 and 7, respectively, are illustrated schematically with an oblique dotted line and they are marked with the same reference numerals as the screen cylinders of Figures 1, 4, 6 and 7, respectively. After the first screening step A, acceptance passes to the second screening step B, while rejections are removed from the pressure screen through channel 12. In the second screening step B, rejections are returned to the suspension to be screened, e.g. into the input channel 8 or the first inlet space VI. In the solution, the incoming fiber suspension can first be roughly screened in a screening step where the silver perforations are larger than usual, with less acceptance than previously removed with the reject, while the first screening step has a good capacity. The second screening step B, for its part, utilizes a normal size perforation size, whereby acceptance found in the reject returned to step A for screening is not lost, but is recovered in the pressure screen. The acceptance removed from the second screening step B is of high quality and contains significantly smaller knits or fiber bundles than before, as the knits and fiber bundles are effectively removed from the suspension during the two steps.

The functional diagram of the pressure screen shown in Figure 3 is otherwise similar to that shown in Figure 2 except that it has a screw press 12a (not shown in Figure

1) mounted at the inlet end of the reject outlet duct. The screw press dewateres the reject. Rejects with a density as high as 20% or even higher are removed through channel 12c and the filtrate is passed through channel 12b back to the material inlet or inlet space VI.

Figure 4 shows another embodiment of the pressure screen according to the invention, where the same reference numerals as in Figure 1 are used for corresponding parts. In the pressure screen of Figure 4, fiber suspension is introduced through an inlet channel 15 which is connected to a screen, i.e. a third inlet space V8 arranged at the other end

515 574 of the pressure screen, in the upper part of the figure. The inner surface of the inlet space V8 is defined by a third screen cylinder 16, the upper end of which is closed with a cover plate 16a to prevent material flow therethrough. The material fraction passed through the screen cylinder 16, i.e. acceptance is led directly to the second screening step, ie. through the second intermediate space V4 into the second inlet space V5. Rejects from the sealing step are passed through a separate conduit Π extending beyond the pressure screen into the first inlet space VI, which belongs to the first sieving stage, from which acceptance is passed through the first sieve cylinder 9 into the first intermediate space V2 and further along the inside of the rotor 3 through the pump space V3 to the pump blades 5. The pump blades for acceptance through the second intermediate space V4 into the second inlet space V5, which belongs to the second screening stage. From the second inlet compartment V5, acceptance is passed through the second screen cylinder 10 into the outlet compartment V6 and removed through the channel 11. Rejects are again fed from the second inlet compartment V5 into the first inlet compartment VI, where it is mixed with the suspension present there, shall be resealed in the screen cylinder 9. The final reject is removed from the first inlet space VI through the outlet channel 12. The screen has additional foil blades 18 connected to the rotor and arranged on the outer surface of the third screen cylinder 16. As the rotor 3 rotates, the foil blades 18 sweep the surface of the screen cylinder 16, thus releasing rejections and other contaminants from the surface in a known manner. Except for the seals 20 and 23 shown in Figure 1, the pressure screen shown in Figure 4 further shows a third seal 19, which seals the rotor and cover plate 16a relative to each other so that fiber suspension should not flow between them, without always flowing through the screen cylinders. Figure 4 shows no rotary mechanism or motor for the rotor as they are already partial

515 574 has been shown in Figure 1 and as such are generally known.

Figure 5 shows a functional diagram of the pressure screen shown in Figure 4. Fiber suspension is introduced into the screen through the channel 15 to the screen stage C, from which acceptance is directed directly to the second screen stage B. Rejects are led from the screen stage C through the channel 17 to the first screen stage A, from where acceptance in the same manner according to the invention is led to the second screening step B. From the screening stage B, acceptance flows out through the channel 11, and the reject is returned to the screening step A, where it is again mixed with new reject from the seal and resilient with it. Final reject is removed through channel 12. As shown in Figure 5, the apparatus may further exhibit a screw press 12a (not shown in Figure 4) which extracts liquid from the reject, and the filtrate is then returned through a channel 12b to the material inlet either through the channel 15 or, if desired, directly to the the cover C, e.g. through a connection 13 shown in Figure 5. Rejects with a density as high as 20% or even more are removed through channel 12c.

Figure 6 shows a solution which functionally corresponds to the pressure screen shown in Figures 4 and 5. This solution avoids the use of seals 19 and 20, while still providing equal sieving capacity. In this case, the foil blades 18, which polish the screen cylinder 16 in the screening stage C, are arranged in the gap V4 instead of the inlet space V8, and the cover plate 16a at the end of the screen cylinder 16 closes the gap completely, and therefore no seal 19 according to Figure 4 is needed. The foil blades 18, in turn, are supported at the upper end by support means 18a. Similarly, the screen cylinder 9 can be arranged to rotate with the rotor in the screening step A using a cylindrical capsule 9a and a connecting annular member 9b, the screen cylinder and the rotor being easily sealed relative to one another by seals 21 and 22. Thereby, the foil blades 6, which polish the screen cylinder 9, are fixed in the screen body so that they form a piece, thereby avoiding the seal which in the solutions shown in figures 1 and 4 is indicated by the reference numeral 20.

Figure 7 shows yet another embodiment of the screen and Figure 8 shows a functional diagram for this embodiment. In this embodiment, the direction of flow in the strainer is opposite to that in the strainers shown in Figures 1, 4 and 6, ie. fiber suspension in this case flows from the bottom up. In this embodiment, fiber suspension is passed through an inlet channel 15 into the third inlet space V8, from which acceptance passed through the third screen cylinder 16 is removed into the annular second intermediate space V4 and further directly out through a channel 25. Reject passes in its side from the inlet space V8. to the screening step B, i.e. into the second inlet compartment V5, from which reject flows through the second screen cylinder 10 into the annular outlet compartment V6 and further out through the duct 11, the ducts and 25 being connected to each other by means of external pipes not shown. From the second inlet compartment V5, the reject rejects into the first inlet compartment VI and further through the first screen cylinder 9, the remaining reject being removed through the outlet channel connected to the first inlet compartment VI. The material entering the inlet compartment VI can be further diluted with water which is fed from a water inlet channel 24 connected to the inlet compartment. The material passed through the first screen cylinder 9 is passed through the intermediate space V2 in the rotor forwards into the pump space V3, from which the pump blades 5 again lead it into the second inlet space V5 for re-screening. In Figure 7, the first screen cylinder 9 is also arranged to rotate with the rotor. Thus, the foil sheets are fixed. A solid one

515 574 closure cap 9c is provided at the end of the screen cylinder

9th The lid prevents flow of fiber suspension through the end of the screen cylinder into it and thus into the rotor. This solution also avoids the use of all annular seals between the screen cylinders and the other components except for the normal shaft seal 23 of the rotor. The functional safety of the device is thus good and the material flows in an appropriate manner.

Figure 8 schematically describes the function of the screen shown in Figure 7. The figure shows how fiber suspension is introduced through the channel 15 to the screening stage C, from where acceptance is removed through the screen cylinder 16 into the intermediate space V4 and further out through the channel 25. Rejects are led to the screening stage B , from where acceptance is removed through the screen cylinder 10 into the space V6 and further forward through the channel 11. Correspondingly, rejections from space V5 to screening stage A are passed into the respective inlet space VI, from which acceptance is removed through the screen cylinder 9 into the space V2 and further is returned into the inlet space V5 in the screening stage B. , although the construction is not shown in Figure 7 and Figure 4, respectively.

The invention has been described above and shown in the drawings by way of example and is not in any way limited to these embodiments, but the scope of protection is defined in the appended claims. Although the foil blades in the embodiments according to the figures are arranged on the screen cylinder surface only on the feed side of the material, foil sheets can also be arranged to rotate along the outlet surface of the screen cylinders and both on the inlet and outlet side of the surface without affecting the scope of the claims in any way. Although the pressure screens in the figures are shown in the vertical direction, they can be mounted in the vertical or horizontal direction. The main current direction of fiber suspension can also be arranged to be either from the bottom up or down from the top, depending on the available space and other requirements.

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