AU4274796A - Screening arrangement - Google Patents

Description

Screening arrangement

This invention relates to an arrangement for separating light impur¬ ities from pulp suspensions. Light impurities are understood to be particles or aggregates with lower density than water, for example plastic particles. Aggregates are found, for example, at flotation where a heavy particle is bound to a gas, and the aggregate particle/ bubble behaves like a light particle.

Heavy impurities in the pulp are separated in conventional screens by means of centrifugal force in separate scrap taps, while coarse fiber fractions and impurities are separated by a screen member whereby the pulp is divided into accept and reject.

Light impurities are more difficult to separate, especially at high pulp concentrations. The pulp concentration, however, is desired to be high, for example 3-5%, in order to achieve a high production capacity and to avoid the transport of large liquid volumes in the screening system.

Light impurities, therefore, usually are separated in separate dev¬ ices, such as hydrocyclones. This requires not only separate equip¬ ment, but also low concentrations, below 1%, in order to achieve a good effect. It is, therefore, a less attractive method to vortex clean the entire flow from a pulp line. It is, instead, possible to carry out pressure screening through very fine slits at highest possible concentration and thereafter to use vortex cleaner for separating the light impurities from the reject flow of the screens.

The present invention offers a solution for the aforesaid problems, in that a device for removing light impurities is designed to be directly connected to the casing of a fractionation means, for ex¬ ample a conventional screen arrangement. The characterizing features of the invention are apparent from the attached claims. The invention is described in greater detail in the following, with reference to the accompanying Figure, illustrating an embodiment of the invention where the separation device is combined with a fract- ionation device in the form of a pulp screen.

The arrangement shown comprises a pulp screen with an airtight cas¬ ing 1, which has an inlet 2 for inject and outlets 3 and, respect¬ ively, 4 for accept and, respectively, reject. A rotation symmetrical screening member 5 with vertical axle is located in the casing 1. The pulp inlet 2, which preferably is tangential, communicates with the inside of the screening member 5 at the upper end thereof while the reject outlet 4 communicates with the lower end of the screening member. The accept outlet 3 is connected to a space 6, which is loc¬ ated outside and extends about the screening member 5. In connection to the upper pprtion of the casing, a space with outlet 7 for heavy impurities, such as sand and scrap, is located.

A rotor 8 within the screening member 5 extends along the entire screening member. The rotor is concentric with the screening member in such a way that a screening zone 9 extending all about is formed between the rotor and screening member. The rotor 8 and screening member 5 can be cylindric or conic.

The rotor 8 preferably is provided with wing elements 10, which are intended to bring about pulsations in the pulp in the screening zone 9 in order to break up the fiber network and to render it poss¬ ible to divide the pulp into accept and reject.

A member 11 for the separation of light impurities is connected cent¬ rally to the upper portion of the casing 1 and communicates with the inferior of the casing 1 via a passage 12. The said member is formed with a rotation symmetrical housing, which preferably comprises a conic upward diverging lower portion 13 and a substantially cylindric upper portion 14. In the transition between the lower and upper hous- ing portions 13, 14, an inlet 15 for dilution liquid is connected tangentially, and an outlet 16 for light impurities is located centr¬ ally at the top of the housing 11. In said outlet 16 a valve 17 is located, for example a sluice valve, for controlling the outflow from the member 11.

A rotor axle 18 can possibly be provided to extend from the casing 1 through the passage 12 upward in the housing 13,14, and a rotor element 19 with a carrier be attached to the axle 18 in the housing 13,14. The rotor axle 18 can be provided with a screw thread 20 for upward feeding. The rotor axle 18 preferably is driven by being attached to the rotor 8 of the fractionation device. The rotor element 19 preferably shall be located on the same height as the inlet 15 for dilution liquid so that correct flow geometry is obtained. The rotor element supplies energy, which otherwise must have been supplied by the inlet rate and amount of dilution liquid. By using the rotor element, thus, the dilution amount supplied can be reduced at maintained separation effectiveness.

The pulp suspension is supplied through the inlet 3 in the casing 1 where it is caused to rotate due to the tangential supply and the rotation of the rotor 8. Scrap and other heavy impurities are coll¬ ected in the space 7 by the effect of centrifugal force. The pulp is introduced into the screening zone 9 and moves axially downward to the reject outlet 4 while being simultaneously rotated. The accept passes thereby through the apertures of the screening member 5. The wing elements 10 bring about pulsations in the pulp which facilitate the division into accept and reject. The reject is discharged through the outlet 4.

Due to the rotation of the pulp in the casing 1, the.light impurities are collected centrally at the top of the casing. The location and design of the separation device 11 give rise to an upward directed flow centrally in the passage 12 at the same time as a downward dir¬ ected return flow is caused in the outward portion of the passagS 12. The light impurities are thereby moved up through the passage 12 to the member 11 where they are collected centrally upwardly. By tang¬ ential supply of dilution liquid through the inlet 15 energy is suppl¬ ied which drives the rotation and produces suitable flow conditions to enrich the impurities and promote the collection of light impurities centrally upwardly in the device 11. These impurities can thereby be taken out through the outlet 16. The discharge, which is controlled by a valve 17, can be continous or intermittent, depending on the amount of impurities. When the arrangement comprises a rotor axle 18 with arotor element 19, the rotation of the contents in the housing 13,14 is affected additionally in that the effect of the tangential dilution liquid supply is increased.

The flow rate through the passage 12 shall be low, of the magnitude 0,02 m/s. The conical design of the lower portion 13 of the device 11 promotes in this portion circulation, which is directed upward at the centre and downward along the conical walls, at the same time as there is a horizontal rotation movement in the entire device 11. Owing to the location of the tangential inlet 15 for dilution liquid, an opp¬ osed circulation movement is caused in the upper portion 14, i.e. a movement directed downward at the centre and upward along the outer walls, which results in a movement of the light impurities toward the centre in this portion 14.

In the embodiment shown the separation device 11 for light impurities is shown connected to a type of screen arrangement, but it is obvious that also other types of fractionation means can be applied, for ex¬ ample screens with rotating screen members and screens with other types of rotors. The screening can take place from the inside outward or from the outside inward through the screen member. The screening can also take place during the passage from above downward or from below up¬ ward through the screening zone. In all cases is common, that the pulp suspension shall be rotated in the upper portion of the casing of the fractionation device.

The invention, of course, is not restricted to the embodiments shown, but can be varied within the scope of the invention idea.

Claims (5)

Claims

1. An arrangement for separating light impurities from pulp sus¬ pensions, intended to be connected to a casing (1) of a fractionat¬ ion device where the fiber network is broken up and the pulp is caused to rotate for enriching the light impurities, c h a r a c t e r i z e d i π that the separation arrangement (11) is designed with a rotation symmetrical housing (13,14) intended through a passage (12) to be conn¬ ected centrally to the upper portion of the casing (1), that an inlet (15) for dilution liquid is connected tangentially to the housing (13,14), and that an outlet (16) for the light impurities is located centrally at the top of the housing (13,14).

2. An arrangement as defined in claim 1, c h a r a c t e r i z e d i n that the housing (13,14) consists of an upward diverging lower portion (13) and a substantially cylindric upper portion (14), and the dilution liquid inlet (15) is located in the passage between the όower and upper portion.

3. An arrangement as- defined in claim 1 or 2, c h a r a c t e r ¬ i z e d i n that the division of the housing into a lower and, respectively, upper portion (13 and, respectively, 14) is determined from a flow aspect of the location of the dilution liquid inlet (15) .

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 a valve (17) is located in the outlet (16) for controlling the discharge of light impurities.

5. 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 rotor axle (18) with a rotor element (19) extends through the passage (12) from the casing (1) of the fractionation device into the housing (13,14) to affect the rotation of the contents in the housing.

5. An arrangement as defined in claim 5, c h a r a c t e r i z e d i n that the rotor axle (18) is provided with a screw thread (20) for upward feeding.