SCREENING ARRANGEMENT

Abstract: An arrangement for screening pulp suspensions, comprising a casing (1) with an inlet (5) for inject and outlets (16, 18) for accept and reject. Between a stationary screening member (2) located in the casing (1) and a drum-shaped rotor (3) located inside the screening member, a screening zone (4) is formed. The inject inlet (5) is provided for the supply of the pulp to the lower portion of the inside of the rotor (3). At least one opening (6) is provided in the upper portion of the rotor (3) for transferring the pulp to the screening zone (4). The inside of the rotor (3) is formed for moving heavy impurities to a chamber (8, 13) in connection to one end of the rotor (3).

Description: Screening arrangement This invention relates to an arrangement for screening pulp suspens¬ ions in order to separate impurities and other pulp fractions, which are not desired to be included in the final product, such as coarse particles, undefibered material and poorly worked fibers. At the making of fiber suspensions, undesired coarse particles, such as,for example, undefibered material, bark, knots etc. , are obtained in the suspension as a result of incomplete manufacturing processes. Also other impurities, both light and heavy ones, such as plastics, sand and scrap, can be found in the suspension. Especially fiber suspensions of slushed return fibers contain great amounts of foreign impurities Heavy impurities, such as stones, sand and glass, plaster and wire clips, and light impurities, such as certain plastics, agglom¬ erated glue lumps ("stickles") etc, , can cause interruptions of the screening process. It is, therefore, desired to separate them at an early stage of the screening. For this purpose, special devices, for exampleknot screens or refiners, can be arranged before the screen in order to eliminate the coarse impurities or reduce their size. Light impurities can also be separated by special devices before the screen. It is also possible to separate the coarse impurities by a first screening step in the screening arrangement or to separate scrap and heavy particles when the suspension enters the screen, and other impurities at the fine screening. In the lastmentioned case, the screening process can be disturbed by the impurities, as mention¬ ed above. At the screening of pulp suspensions it is also desired to have a high pulp concentration, for example 3-5%, and a low reject draw-off in order to achieve a high production capacity and to avoid unnecess¬ arily large liquid transports in the screening system. High concen¬ tration and low reject draw-off, however, imply greater difficulties to separate the impurities from the pulp. 4 able way as mentioned above. In this way, a separation of heavy impur¬ ities is achieved in a first step. At the embodiment shown in Fig. 2, the inside of the rotor 3 prefer¬ ably is cylindric and possibly provided with strips 15 ' on-grooves axial with or at a small angle to the axial direction, so that the heavy impurities are guided with the pulp flow upward along the inside of the rotor 3 to be discharged to a chamber 13 intended for this purpose at the upper edge of the rotor 3. According to this embodiment, the openings 6 in the upper portion of the rotor 3 for transferring the pulp to the upper end of the screening zone 4 are formed with an edge 14 extending a distance inward from the inside of the rotor 3. The coarse and heavy impurities are hereby prevented from moving upward along the inside of the rotor to follow along with the pulp through the openings 6. These impurities, instead, are guided past the openings 6 to the chamber 13, from which they can be taken out discontinuously or continuously, depending on the expected content of impurities in the pulp. At the same time as heavy impurities are concentrated at the inner surface of the rotor 3, light impurities are concentrated at the wall surface of the housing 20. These light impurities are guided upward and accumulate centrally upward in the rotor 3, from where they can be discharged. The light impurities, for example, can be guided up¬ ward through the rotor top and be discharged centrally from the upper portion of the casing 1. For this purpose, the wall of the housing 20 can be formed with strips or grooves in order to promote the separ¬ ation of the light impurities. The separation of heavy and, respect¬ ively, light impurities, thus, takes place before the pulp enters the screening zone 4. The pulp flow flowing through the openings 6 at the top of the rotor 3 continues downward in the screening zone 4 for fine screening in a second step whereby the pulp is divided into accept and reject. This dividing of the pulp into accept and reject is promoted there by the pulsation generating means 7, which bring about pressure and speed variations in the pulp suspension which are favourable for the screening. Due to the accept passing through the screening member At the embodiment shown in Fig. 1, the inside of the rotor 3 is formed for moving heavy impurities downward to a chamber 8 in connection to the lower portion of the rotor 3, and the lower edge of the rotor 3 extends downward into the chamber 8. In order to bring about this separation, the inside of the rotor can be conical with the greatest diameter lowermost. The inner surface can be smooth or possibly be provided with strips or grooves extending axially or angularly to the axial direction. This angle can be small or so that the strips extend in screw form about the rotor. When strips or grooves are used, the cone angle can be smaller. It is also possible to form the rotor inside cylindric. The stationary wall of the housing 20 can be formed in a corresponding manner for an upward discharge of light impurities. At both embodiments shown an inlet 9 for dilution liquid is connected to the casing 1 of the screening arrangement. This inlet communicates with a space 10 in the rotor 3, which space is formed with openings 11 in the rotor for the supply of dilution liquid to the screening zone 4, preferably in the lower portion of the screening zone. At the embodiment shown in Fig. 1, between the inlet 9 for dilution liquid and the chamber 8, a passage 12 is located, through which a restricted amount of dilution liquid can pass. This passage 12 preferably is provided between the lower edge of the rotor 3 and the upper defining wall of the chamber 8. The chamber 8 can be designed for discontinuous or continuous empty¬ ing, depending on the expected content of heavy impurities in the pulp. At the embodiment shown in Fig. 1, pulp to be screened is supplied through the inlet 5 to the inside of rotor 3. At the same time as the pulp is rotated by the rotor, it flows upward through the rotor to the openings 6 at rotor top. Owing to the effect of centrifugal force, heavy impurities are collected adjacent the inner surface of the rotor. Due to the rotor design, these impurities are guided downward to the chamber 8, from where they can be removed in a suit- 6 Claims 1. An arrangement for screening pulp suspensions, comprising a cas¬ ing (1) with an inlet (5) for inject and outlets (16,18) for accept and reject, a stationary screening member (2) located in the casing (1) and a drum-shaped rotor (3) located inside the screening member whereby between the rotor and screening member a screening zone (4) is formed, c h a r a c t e r i z e d i n that the inject inlet (5) is provided for the supply of the pulp to the lower portion of the inside of the rotor (3) , that at least one opening (6) is provided in the upper portion of the rotor (3) for transferring the pulp to the screening zone (4) , and that the inside of the rotor (3) is formed for moving away heavy impurities to a chamber (8,13) in connection to one end of the rotor (3) . 2. An arrangement as defined in claim 1, c h a r a c t e r i z e d l ran that the chanter (8) far discharging heavy impurities is located in connection to the lower edge of the rotor (3) . 3. An arrangement as defined in claim 1, c h a r a c t e r i z e d l n that the chamber (13) for discharging heavy impurities is located in connection to the upper edge of the rotor (3) . 4. An arrangement as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the inside of the rotor (3) is cylindric and provided with strips (15) or grooves, which are axial or form an angle to the axial direction. 5. An arrangement as defined in claim 4, c h a r a c t e r i z e d l n that the inside of the rotor (3) is provided with strips or grooves, which extend in screw form about the rotor. 6. An arrangement as defined in claim 2, c h a r a c t e r i z e d l n that the rotor (3) is formed with a conical inner surface with downward increasing diameter. together with a part of the liquid, the liquid content in the reject transported along the screening zone 4 decreases. This thickening of the reject is counteracted by the supply of dilution liquid through the openings 11 in the rotor 3 at the end of the screening zone. The supply of dilution liquid preferably is controlled so that the out¬ going reject has the desired concentration. For taking out the accept, an accept outlet 16 is connected to a space 17 in the casing 1, which space is located outside the screening member 2. A reject outlet 18 is connected to the casing 1 for discharging the reject after the screening zone 4. The invention, of course, is not restricted to the embodiments shown and described, but can be varied within the scope of the invention idea.

Claims: 7. An arrangement as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the inject inlet (5) is loc¬ ated tangentially in relation to the rotor (3) . 8. An arrangement as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that a stationary housing (20) with a rotation symmetrical wall is located within the rotor (3) , which wall extends axially spaced from the inside of the rotor. 9. An arrangement as defined in claim 8, c h a r a c t e r i z e d i n that the wall of the stationary housing (20) is provided with strips (19) or grooves for discharging light impurities.

Inventors: FREDRIKSSON BOERJE, FREDRIKSSON, BOERJE
Applicants: FREDRIKSSON BOERJE, SUNDS DEFIBRATOR IND AB, FREDRIKSSON, BOERJE, SUNDS DEFIBRATOR INDUSTRIES AB
Application year: 1996
Main classification: D21D5
IPC codes: B01D29, D21D5