During operation a first fraction of the incoming liquid mixture passes the screen member, while a second fraction of the liquid mixture develops in the chamber. The screening apparatus further comprises a pulse generating device for generating pulses that counteract clogging of the screen holes of the screen member, and a drive means for providing relative displacement between the pulse generating device and the screen member.

Especially within the paper pulp industry screening apparatuses of this kind are used for a number of different applications. One application is screening of relatively heavily polluted pulp suspensions whith the aid of a course screening system containing several interconnected stages of screening apparatuses. Another application is screening of relatively slightly polluted pulp suspensions with the aid of a fine screening system likewise containing several inter- connected stages of screening apparatuses. In these cases each screening apparatus separates the pulp suspension so that the first fraction passing through the screen member substantially contains fibres while the second fraction contains pollutions and some fibres. For the course screening system as well as the fine screening system it is important that the final stage has a good separation efficiency, in order to achieve a satisfactory separation efficiency of the entire screening system. Thus, if the separation efficiency of the final stage is poor, a share of the pollutions is returned to the pulp suspension while a share of the fibres

is lost with the rejected pollutions.

The screen holes of the screening apparatus may of course be dimensioned relatively small, so that at least the separation efficiency with respect to the pollutions will be satis- factory. However, this would result in a decreased fibre yield, since a significant amount of fibres would be lost with rejected pollutions. Besides, the resulted increased total amount of reject would increase the dumping costs for the reject, which in some countries can be significant. For example, it is not unusual that the costs for dumping a certain amount of reject is equally high as the costs for producing a corresponding amount of recycled paper. Thus, it may be of great economical importance that usable fibres are separated from the pollutions before the latter are removed from the screening system.

Yet another application is recovering of fibres from process liquid circulating in systems for producing pulp and paper.

In this case, the screening apparatus separates the process liquid so that the first fraction passing the screen member contains liquid substantially free from fibres, e. g. usually water, while the second fraction contains detained fibres which are recycled.

A prior screening apparatus of the kind here presented has a screen member in the form of a stationary cylindrical screen basket, against which the liquid mixture to be separated is supplied under a relatively high pressure, and a pulse generating device in the form of a rotating impeller. The impeller comprises wing-like elements that sweep along the inject side of the screen basket to generate suction pulses in the screen passages of the screen basket, so that these are kept open. To generate the suction pulses a chamber surrounding the side opposite the inject side of this screen

basket is kept constantly filled with the fraction of the liquid mixture which has passed through the screen basket, with the aid of a counter pressure maintained in the chamber.

This known screening apparatus has a very high capacity but is relatively expensive and has a relatively poor separation efficiency. Besides, it is unsuitable for separating liquid mixtures containing hard abrading particles wearing the screen basket, in particular if the liquid mixture is supplied centrally into the screen basket. For these reasons, among other things, the known screening apparatus is unsuited for use as a final stage in a course or fine screening system for cleaning pulp suspensions. Traditionally, simpler rela- tively small and expensive screening apparatuses, often having low capacity and poor separation efficiency due to relatively large screen holes, for example vibration screens, have therefore been used as such final stages.

The object of the present invention is to provide a simple inexpensive screening apparatus, which is suited for se- parating liquid mixtures containing hard abrading particles.

Another object of the invention is to provide a simple inexpensive screening apparatus, which has a better se- paration efficiency than the above described known screening apparatuses and which therefore to a relatively low cost is suited for use as a final stage in both fine and course screening systems for separating polluted fibre suspensions.

Yet another object of the present invention is to provide a simple inexpensive screening apparatus, which is suited for recycling fibres from process liquid for pulp and paper production.

These objects are achieved by a screening apparatus of the kind described initially, which according to a first general

aspect of the invention is characterized in that the screen member comprises a circular rotatable screen disc, which is rotatable about its center by a drive means, and that the pulse generating device is adapted to generate pressure pulses against the screen disc downstream thereof, when the screen disc is rotated by the drive means. Since the pressure pulses act against the screen disc at the downstream side thereof there is no need for maintaining a counter pressure in the first fraction developed at the downstream side. Thus, the liquid mixture can be supplied to the screening apparatus according to the invention under a relatively low pressure, which results in the advantage that the screening apparatus does not need to be designed as a pressure vessel, in order to stand high pressures, but can be given a simple and in- expensive design.

Preferably, the pulse generating device is stationary and comprises at least one elongated pulse element extending along and spaced from the screen disc toward the radially outer circumferential end of the screen disc. To achieve an even load on the screen disc the pulse generating device suitably comprises at least two pulse elements symmetrically distributed in the circumferential direction of the screen disc. (Alternatively, the pulse generating device, stationary or moveable, may comprise a number of nozzles for continuous or intermittent spraying of gas and liquid against the screen disc.) Each pulse element may have a surface portion facing the screen disc and forming an attack angle thereto, as seen in the circumferential direction of the screen disc, and may have a propeller blade-like shape, so that the attack angle successively increases along the pulse element in the direction radially inwardly relative to the screen disc.

Hereby, the decreasing velocity of the screen disc relative to the pulse element along the screen disc, as seen in the direction radially inwardly, can be compensated, so that the action of the pulses on the screen disc will be efficient regardless of the radial location where the pulses act upon the screen disc. Alternatively, said compensation may be created by decreasing the distance between each pulse element and the screen disc along the pulse element in the direction radially inwardly relative to the screen disc.

Advantageously, the screen disc is provided with at least one elongated turbulence element, which extend along the screen disc toward the radially outer circumferential end of the screen disc and which projects into the chamber for generat- ing turbulence in the liquid mixture in the chamber during rotation of the screen disc, whereby the occurrence of particles blocking the screen holes of the screen disc is counteracted. To compensate the decreasing relative velocity between the turbulence element and the liquid mixture, as seen in the direction radially inwardly relative to the screen disc, the turbulence element suitably has an axial extension relative to the screen disc which increases along the turbulence element in the direction radially inwardly relative to the screen disc. Preferably the turbulence element extends obliquely to the radius of the screen disc, so that the radially outer end of the turbulence element is ahead of the radially inner end of the turbulence element, as seen in the rotational direction of the screen disc,. Hereby, the action of the turbulence element like an impeller of a centrifugal pump is counteracted.

The screen disc is suitably provided with at least one elongated entrainment element extending along the screen disc toward the outer circumferential end of the screen disc and projecting from the screen disc at the downstream side

thereof for entraining the developed first fraction during rotation of the screen disc. Hereby it is avoided that the first fraction forms a still standing liquid layer between the screen disc and the pulse elements, which in particular may occur if the first fraction contains network forming fibres. Thus, the entrainment elements ensures that the stationary pulse elements generate the intended pressure pulses against the screen disc.

Preferably, the screen disc is provided with an outer circumferential ring forming an annular sealing surface, which extends opposite an annular sealing surface of the housing, so that an annular axial sealing gap is defined between the sealing surfaces of the circumferential ring and the housing. Furthermore, the housing is provided with liquid channels opening in the sealing gap to supply dilution liquid to the gap for lubricating the sealing surfaces, when the screen disc is rotated, and to supply dilution liquid to the chamber. If the second fraction, which is developed in the chamber, contains pollutions these are washed by the dilution liquid supplied to the chamber, so that particles, for in- stance fibres, that would have followed the first fraction are uncovered and are able to pass through the screen disc.

This results in an improved fibre yield when separating fibre suspensions.

The object of the invention is also achieved by a screening apparatus according to the kind described initially, which according to a second general aspect of the invention is characterized in that the housing forms a second opening to the chamber, that the screen member comprises a first circular screen disc covering the first opening, and a second circular screen disc covering the second opening, that each of the first and second screen discs is rotatable about its center by the drive means, and that the apparatus comprises a

first and second, respectively, pulse generating device for generating pressure pulses against the first and second, respectively, screen disc at the downstream side thereof, when the screen discs are rotated by the drive means. Hereby the diameter of the first and second, respectively, screen disc can be reduced compared to the diameter of the single screen disc of the screening apparatus described above according to the first general aspect of the invention. This gives the advantage that the load on each screen disc is reduced. In addition, the advantage is achieved that if any of the two screen discs would be clogged during operation the screening apparatus would still work with the aid of the remaining screen disc, although with a reduced capacity.

According to a preferred embodiment of the invention the first and second openings are situated opposite each other, and the first and second screen discs are interconnected to each other by a common centre shaft. Hereby the advantage is achieved that the loads on the two screen discs created by the differential pressures across these do not need to load any thrust bearing for each screen disc, but the loads are carried by the centre shaft.

The first and second, respectively, pulse generating device may be designed in the corresponding manner as the aboved described pulse generating device of the screening apparatus according to the first general aspect of the invention. In addition, the first and second, respectively, screen disc may be provided with at least one turbulence element and at least one entrainment element of the kind and in the manner as described above in connection with the screening apparatus according to the first general aspect of the invention.

Preferably, the first and second, respectively, screen disc is provided with a first and second, respectively, outer

circumferential ring forming a first and second, respective- ly, annular sealing surface extending opposite a first and second, respectively, annular sealing surface of the housing, so that a first and second, respectively, annular axial sealing gap is defined between the first and second, re- spectively, sealing surface of the circumferential ring and the first and second, respectively, sealing surface of the housing. In addition, the housing is provided with liquid channels opening in the first and second, respectively, sealing gap, for the supply of dilution liquid to the sealing gap for lubricating the sealing surfaces, when the screen discs are rotated, and for supplying dilution liquid to the chamber.

The invention also relates to a circular annular screen disc for use in a screening apparatus according to the first and second, respectively, aspect of the invention, comprising a plurality of screen segments arranged in series around the centre of the screen disc, and spokes connecting the screen segments with one another, each spoke having a radially outer end and a radially inner end. The screen disc is charac- terized in that each spoke extends obliquely to the radius of the screen disc, so that the outer end of the spoke is ahead of the inner end of the spoke, as seen in the rotational direction of the screen disc.

According to an embodiment of the screen disc according to the invention at least one spoke is dimensioned such that it substantially protrudes axially from the screen segments at the side of the screen disc, the upstream side, which is intended to receive the liquid mixture to be separated, whereby the spoke generates turbulence in liquid mixture existing close to the screen disc during rotation of the latter.

According to another emodiment of the screen disc according to the invention at least one spoke is dimensioned such that it substantially protrudes axially from the screen segments at the side of the screen disc which is intended to consti- tute the downstream side of the screen disc. Said one spoke will entrain liquid existing close to the screen disc during rotation of the screen disc.

The invention is described in more detail in the following with reference to the accompanying drawings, in which Figure 1 schematically shows a screening apparatus according to an embodiment of the invention, Figure 2 shows a screen disc for the screening apparatus according to figure 1, Figure 3 shows a detail of the screen disc according to figure 2, Figure 4 schematically shows a sealing device of the screening apparatus according to figure 1, and Figure 5 schematically shows another embodiment of the screening apparatus according to the invention.

In the figures, components with corresponding function have been provided with the same reference numerals.

Figure 1 shows a screening apparatus 2 according to an embodiment of the invention, comprising a housing 4 having a bottom portion 6, which forms a cylindical chamber 8 and a circular horisontall opening 10 thereto, and a top portion 12 connected to the bottom portion 6 by a hinge 14. The top portion 12 forms an annular chamber 16, which communicates with the cylindical chamber 8 through the opening 10 and which freely communicates with the surroundings through an outlet opening 18. A circular horisontal screen disc 20 covers the opening 10 and is rotatable about its centre by a

drive means in the form of an electric motor 22 attached to the top portion 12.

There is an inlet member 24 on the bottom portion 6 for supplying a liquid mixture to be separated substantially tangentially into the chamber 8, so that a first fraction of the liquid mixture passes through the screen disc 20 into the chamber 16 and is discharged through the outlet opening 18, while a second fraction of the liquid mixture is developed in the chamber 8. An outlet member 26 on the bottom portion 6 may be adapted to continuously or, alternatively, inter- mittently which is preferred, discharge the developed second fraction from the chamber 8.

The screen disc 20 comprises a plurality of screen segments 28, here eight screen segments 28, provided with screen holes and arranged in series around the centre of the screen disc 28, and spokes 30a and 30b interconnecting adjacent screen segments 28, see figure 2. Each spoke 30a, 30b extends obliquely to the radius of the screen disc 28, so that the radially outer end 32 of the spoke 30a, 30b is ahead of the radially inner end 34 of the spoke 30a, 30b, as seen in the rotational direction of the screen disc 28 indicated by an arrow in figure 2. Alternatively, the spokes 30a, 30b may extend radially.

Every second spoke 30a forms an elongated turbulence element 36, see figure 3, protruding into the chamber 8 for generat- ing turbulence in the liquid mixture during rotation of the screen disc 20. Each turbulence element 32 has an axial extension relative to the screen disc 20 which inreases along the turbulence element 32 in the direction radially inwardly relative to the screen disc 20. The other spokes 30b form elongated entrainment elements 38 protruding in the chamber 16 for entraining the developed first fraction during

rotation of the screen disc 20. Alternatively, each spoke 30a, 30b may form a turbulence element 32 and an entrainment element 38.

The screening apparatus 2 is provided with a stationary pulse generating device 40, which is attached to the top portion 12 and which extends in the chamber 16 closed to but spaced from the screen disc 28. The pulse generating device 40 comprises a plurality of elongated pulse elements 42 extending sub- stantially radially along the screen disc 20 and symmetri- cally distributed in the circumferential direction of the screen disc 20. Each pulse element 42 has a surface portion facing the screen disc 20 and forming an attack angle thereto, as seen in the circumferential direction of the screen disc. The surface portion of the pulse element 42 has a propeller blade-like shape, so that the attack angle successively increases along the pulse element 42 in the direction radially inwardly relative to the screen disc 20.

Instead of having a propeller blade-like shape, each pulse element 42 may be arranged such that the distance between the pulse element 42 and the screen disc 20 decreases along the pulse element 42 in the direction radially inwardly relative to the screen disc 20.

In figure 4 there is schematically shown how the rotating screen disc 20 is sealed against the top portion 12. The screen disc 20 is provided with an outer circumferential ring 44 forming an annular sealing surface 46, which is parallel to the screen disc 20. The top portion 12 comprises a wear ring 46 with an annular sealing surface 48 extending opposite the annular sealing surface 46 of the circumferential ring 44, so that an annular axial sealing gap 50 is defined between the sealing surfaces 46 and 48. In the top portion 12 there are liquid channels 52 opening in the sealing gap 50 for supplying dilution liquid thereto.

During operation of the screening apparatus according to figures 1-4, the liquid mixture to be separated, for example a polluted fibre suspension, is pumped tangentially into the cylindrical chamber 8 via the inlet member 24, which results in that possibly occurring heavy pollutions separate radially outwardly in the chamber 8, while an accept fraction of the fibre suspension substantially containing fibres passes through the screen disc 20, which is rotated by the electric motor 22, into the chamber 16. The rotating turbulence elements 36 of the spokes 30a generate turbulence in the chamber 8 in close vicinity to the screen disc 20. The generated turbulence counteracts the formation of fibre network that may reduce the flow through the screen disc 20.

The entrainment elements 38 rotate with the rotating screen disc 20 and serve to entrain the developed accept fraction in immediate vicinity to the screen disc 20 past the stationary turbulence elements 42 so that the latter generate pressure pulses in the accept fraction directed against the screen disc 20. The pressure pulses push out possibly blocking particles from the holes of the screen disc 20 back to the chamber 8.

Dilution liquid continuously pumped into the liquid channels 52 is conducted to the sealing gap 50 where the dilution liquid forms a lubricating and sealing liquid film between the stationary sealing surface 48 and the moving sealing surface 46 of the circumferential ring 44. From the sealing gap 50 dilution liquid is pressed by the rotation of the screen disc 20 radially outwardly into the radially outer part of the cylindical chamber 8 where the dilution liquid washes accumulated pollutions, so that fibres are released and may pass through the screen disc 20. At regular intervals the supply of fibre suspension to the chamber 8 is inter- rupted and the outlet member 26 is temporarily open with the aid of a valve, not shown, so that the dilution liquid washes

the chamber clean of accumulated pollutions, which are discharged via the outlet member 26. Alternatively, by the outlet member 26 it is also possible to continuously discharge a relatively small flow containing enriched pollutions from the chamber 8.

Figure 5 shows a screening apparatus 53 according to another embodiment of the invention, basically differing from the embodiment according to figure 1-4 in that a further circular screen disc 54 of the same kind as the screen disc 20 is arranged. The screen disc 54 covers a further opening 56 existing between the chamber 8 and a further chamber 58 formed in the bottom portion 6 of the housing. The chamber 58 communicates with the surroundings by an outlet opening 60.

The screen disc 54 is coaxially connected to the screen disc 20 by a common centre shaft 62, whereby both screen discs 20, 54 are driven by the electric motor 22. In the chamber 58 there are also stationary pulse elements 64 of the same kind as the pulse elements 42. The bottom portion 6 is provided with liquid channels 66 of the same kind as the liquid channels 52 for supplying dilution liquid between sealing surfaces of the bottom portion 6 and the screen disc 54.

The screening apparatus 53 works in the same manner as the above described screening apparatus 2 except that also the screen disc 54 participates in the separation of the incoming liquid mixture.