Secondary Knot Screens

Secondary knot screening is a barrier screening application targeted at the recovery

of good fibers from the knot stream coming from the primary knotter. Modern

secondary knot screens are equipped with a screw rotating inside a vertical or

inclined perforated screen cylinder. The pulp feed enters the screen near the bottom.

As the knots are transported upwards by the screw, accepted fibers pass

Fig. 6.38 The Noss Raditrim [38].

604 6 Pulp Screening, Cleaning, and Fractionation

through the screen apertures to the annular accept chamber. Shower liquid is

added to the knots, and assists separation by washing good fibers from the knots

to the accept side. A certain liquor level is maintained inside the screen, and after

the knots emerge from the liquor they dewater by gravity before being discharged

through the reject nozzle. The enclosed design of such secondary knot screens

avoids emissions to atmosphere.

The Noss Raditrim is an example of a vertical atmospheric knot screen

(Fig. 6.38). In line with other manufacturers’ screen designs, shower liquid is

introduced into the lower end of the screw shaft and becomes distributed through

holes in the shaft.

Secondary knot screens are typically fed with an inlet consistency between 1.0%

and 1.5%, and deliver knots at a consistency of 25–30%. The amount of good

fibers carried along with the knots is in the range of 10% of the total reject.