It is known that variations in the concentration of the pulp suspension are of decisive importance for the screening process. A reduction in the concentration implies an increase of the hydraulic load on the screen means, i. e. the flow rate through the orifices in the screen means increases. At concentrations below about 0,5 % the capacity becomes unacceptably low. An increase in the concentration implies an increase of the energy intensity required for breaking up the fiber network to individual fibers and causes it to become fluid, so-called fluidization, which is a prerequisite for the screening process. The concentration, therefore, sets a limit for an efficient utilization of the screen. As a result of too high a concentration, the flocks of the pulp will not be broken up, which implies that the screeting process cannot continue.

In a conventional pressurized screen for pulp suspensions, the thickening along the length of the screening zone is the physical problem limiting the effectiveness of the screen with regard both to capacity and efficiency. From a physical point of view, the thickening implies that the concentration of the fiber suspension increases from the inlet to the reject outlet along the surface of the screen basket. Increased concentra- tion implies that the strength of the fiber network increases considerably.

Due to the fact that the rotating means of the screen rotate at equal speed along the entire length of the screening zone, the energy supply is substantially constant from the inject end to the reject end of the screen means. This implies that the screening must start at too low a concentration at the beginning of the screening zone, in order to prevent the pulp concentration from rapidly becoming so high, that a large portion of the screening zone operates as a thickener. An energy intensity too high in relation to the pulp concentration implies that the fiber suspension is overfluidized at the beginning of the screening zone, which yields an unnecessarily high turbulence level and, as a result thereof, a deteriorated separation selectivity. After a short zone with ideal conditions, the pulp concentration will be too hui-, the energy is not any longer sufficient for breaking up the fiber network, and the final portion of the screening zone operates as a thickener. In other words, the thickening implies that the screen looses efficiency and capacity.

At certain modern pulp screens one has succeeded to increase the pulp concentration by providing inside the screen means a rotor with pulsation-creating wings, which yield an extended suction pulse producing a vacuum adjacent the screen means, in order in this way to recover a certain amount of the liquid lost by the thickening.

At the same time, overpressure arises on the inside of the pulsation wings. Extended suction pulses by wide pulsation wings render it possible to increase the concentration in a screen, but at this high concentration the process according to the above reasoning becomes very sensitive from an optimization aspect. Small variations in the pulp con- centration, dewatering properties or fiber length distribution affect the critical balance between network strength and energy supply. As a result thereof, one is forced to operate the screen at a number of revolutions higher than at optimum in order to manage the operability even at normal process variation. Especially at the end of the screening zone the effect of the suction pulses tends to diminish, with thickening problems resulting thereof.

According to the present invention, the above problems can be reduced substantially by forming the screen with dilution liquid supply at the reject outlet through nozzles directe into the screening zone. The variations in thickening arising at normal conditions can thereby be counteracted effectively, especially at changes in production and quality. It is then possible to increase the ingoing and outgoing concentration of the screen, its capacity and efficiency, and to lower the energy consumption. The dilution liquid supply according to the invention has the essential avantage that it permits to drive the screen at a lower number of revolutions.

The characterizing features of the invention are apparent from the attache claims.

The invention is described in greater detail in the following, with reference to the accompanying drawing illustrating an embodiment of the invention.

Fig. 1 is a schematic cross-section of a screening device according to the invention; Fig. 2 is a cross-section according to II-II in Fig. 1; Fig. 3 shows a detail of the screening device with an alternative design of the nozzles.

The screening device shown comprises an air-tight casing 1 with inlet 2 for the pulp suspension (inject) and outlets 3 and 4, respectively, for accept and rejet. respectively. In the casing 1 a cylindric screen means 5, preferably with a vertical symmetry axis, is located stationary. In the screen means 5 a rotor 6 is located and extends along the entire screen means. The rotor 6 is concentric with the screen means, so that an overall screening zone 7 is formed between the rotor 6 and the screen means 5. Alternatively, the screen means can be rotary at a speed relative to the rotor 6.

The inject inlet 2 for the pulp suspension is connecte to the casing 1 for the supply of the pulp from below to the inside of the rotor 6. The rotor is formed as a drum, through which the supplie pulp suspension is intended to flow upward and through one or several openings 8 in the upper portion of the rotor 6 for transferring the pulp to the upper end of the screening zone 7. The rotor is on the outside provided with pulsation wings 9, which extend along the entire screening zone 7. These swings 9 are spaced from the rotor and formed with a leading edge 20 located near the screen means 5 and a rear edge 21 located at a greater distance from the screen means. The swings 9 produce thereby an extended suction pulse when they move alonly the screen means 5 which keeps the screen means open and promotes the separation of the accept. The wings 9 can be of the kind shown in SE-PS 464 473. The remaining part of the pulp suspension is moved on to the reject outlet 4.

Due to the separation of liquid from the suspension together with the fibers through the screen means 5, thickening takes place in the flow direction in the screening zone 7. The pulsation wings 9, furthermore, due to their design produce at their rotation an increased pressure and increased concentration inward to the rotor 6.

In order to counteract congestion of the pulp in the lower portion of the screening zone 7 adjacent the rotor 6, a number of stationary nozzles 10 for the supply of dilution liquid are placed at the reject outlet 4 and directe into the screening zone 7 along the surface of the rotor 6. Hereby an upward directe flow adjacent the surface of the rotor 6 is produced. The directe dilution liquid supply, in addition to conter- acting thickening of the pulp at the reject outlet, counteracts an unfavourable pressure balancing between the front side and rear side of the pulsation wings 9 at the lower end of the wings.

The nozzles 10 should be uniformly distributed around the rotor 6 and can suitably be 6-12 nozzles, as shown in Fig. 2. They can be directe axially or obliquely into the screening zone 7. The nozzles should be placed on the same level as the lower end of the pulsation wings 9 and preferably on an overall wall element 11, which at the same time forms a gap seal to the rotor 6 for defining the reject outlet 4 from the inside of the rotor.

In Fig. 3 an alternative design of the nozzles is shown, where they consist of an overall gap 12. The gap can be defined between the rotor 6 and stationary wall element 11. In this case the nozzle gap 12 is defined between a movable and a stationary part. This implies at the same time that the risk of glogging is reduced.

Pulp to be screened is supplie via the inlet 2 to the inside of the rotor 6 and through the openings 8 in the rotor to the screening zone 7, through which the pulp is moved downward from one end to the other. The accept passes through the screen means 5 together with a portion of the liquid, which results in thickening of the reject transported along the screening zone 7. The thickening of the reject is to a certain extent counteracted by the pulsation wings 9. Variations in the thickening of the reject are counteracted by controlled supply of dilution liquid under pressure through the nozzles 10 at the end of the screening zone 7. In addition to counteracting thickening of the reject by dilution, the spraying in of liquid brings about a favorable flow at the reject outlet. The dilution liquid supply preferably is controlled so that the concentration of outgoing reject is held on a desired level.

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