Introduction. It is useful to start with some definitions and to develop a basic understanding of

It is useful to start with some definitions and to develop a basic understanding of

the mechanical set-up of a pressure screen before looking at the mostly mechanistic

and semi-empirical models that are used to describe the process phenomena

in a pressure screen.

Figure 6.1 shows the process streams around a screen. The pulp suspension

containing the impurities is fed with the feed stream QF at the pulp concentration

cF. The clean pulp passes through the apertures in the screen plate and is discharged

with the accept stream QA at the concentration cA. The reject stream QR is

rich in impurities. More in general, one fraction of a certain property is concentrated

in the reject stream QR, while the other fraction of different property leaves

the screen with the accept stream QA.

QF, cF QA, cA

QR, cR

Fig. 6.1 Streams around a screen.

In addition to the streams shown in Fig. 6.1, most industrial pressure screens

have an internal dilution by filtrate near the reject outlet. While dilution is often

needed from an operational perspective to avoid plugging of the screen’s reject

side, we can skip internal dilution during the present theoretical considerations

without losing insight.

The solid-solid separation with a pressure screen can be categorized by the type

of physical separation mechanism. In barrier separation, particles are rejected

from passage through an aperture because they are physically larger than the

aperture size in any orientation. In probability separation the particle can pass

through the aperture in at least one orientation and it depends on how the particle

approaches the aperture whether it passes, or not [1,2].

The distinction between barrier screening and probability screening is of utmost

importance for all screening applications. As for screening efficiency, probability

separation is apparently much more challenging than barrier separation.

The basic mechanical elements in control of separation in a simple pressure

screen are the screen and the rotor. In the most common design, the screen has

the shape of a cylindrical basket and is fixed in the screen housing. The apertures

in the screen basket have the form of either holes or slots and the screen surface

may be smooth or contoured.

6.2 Screening Theory 563

Pulp is fed axially to the screening zone. The accepted material passes through

the apertures in the screen basket, while the rejected material proceeds along the

inside of the screen basket towards the reject outlet. The rotor revolves inside the

screen basket. Over the years, a large number of different rotor designs has been

developed. Most of these are based on the closed bump rotor and open foil rotor

design principle (Figs. 6.2 and 6.3).

Reject

Accept

Reject

Accept

Reject

Accept

Rotor

Screen basket

Feed Feed Feed

Fig. 6.2 Rotor categories: closed rotor (left), open rotor (center), semi-open rotor (right).

Fig. 6.3 Examples of cross-sections of simple rotors:

left, bump rotor;center, step rotor; right, foil rotor.

Besides providing the tangential velocity near the screen and generating turbulence,

the most important task of the rotor is to keep the screen apertures clear.

This is accomplished by the regular backflush through the apertures caused by

the pumping action of the rotor’s pulsation elements.

6.2.2

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