1114 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning

The screening efficiency depends on the following influences [23]:

_ Parameters of the material to be screened (inlet pulp).

– Type of material (mechanical pulp, chemical pulp, recycled pulp)

– Shape of the particles to be separated (cube-like, flake-like,

stretched length directionally)

– Particle size distribution

– Flexibility of the particles

– Drainage resistance and consistency of the inlet slurry

_ Parameters of the screening elements

– Shape of the element (mesh, hole, slit)

– Size of the element (mesh opening, hole diameter, slit width)

– Position and distance of the elements

– Portion of open screening surface

_ Process or design parameters

– Evenness of flow

– Evenness of composition and consistency

– Type of inlet feeding to the screening elements

– Geometry of feeding stream contact with the screening surface

– Speed and length of feeding stream over the screening element

– Transportation force of accept through the screening elements

– Slurry height over the screening surface and specific feed to the

screening surface

– Mechanical measures to prevent plugging of the screening elements

surfing mater

The coarse rejects in the grinding process are separated in the coarse screening

using vibration screens (Fig. 5.3) with holes of 6-mm diameter. The flow rate is

between 40 and 50 t day–1 at a feeding consistency of 1% and a screen area of

1.5 m2.

Fig. 5.3 Bull screen (vibration screen). 1,Groundwood feed

from grinder pit; 2, screen plate; 3,shower water; 4,coarse

reject; 5,accept.

5.2Machines and Aggregates for Screening and Cleaning 1115

The coarse reject is further treated in shredders. In pressure grinding, this

shredder treatment is applied to the whole pulp before pressure relaxation.

In refiner mechanical pulping, there is virtually no such coarse material in the

pulps (TMP, CTMP, and CMP) any longer. These pulps contain more mini-shives,

chops and long fibers than can be separated in fine screening steps, and are

further treated by reject refining.

Pressure screens are the most common equipment in screening and fractionation

of mechanical pulp (Fig. 5.4, left). The pressure screen consists of a cylindrical

screen basket as a screening element, and a concentric positioned rotor to

keep the screen openings unplugged (see also Fig. 5.5). The pulp suspension is

fed from above, either axially or tangentially, with the flow operating either centrifugally,

centripetally, or as a combination of the two. Either the rotor or the screen

basket can rotate, but today the major application is centrifugal flow with a rotor.

Single-stage screens operate with low-consistency or medium-consistency pulps,

whereas in multistage screens several stages are run in one screening apparatus

(Fig. 5.4, right). In future, these screens will increasingly replace existing singlestage

screens. The shives and stiff long fibers are removed as rejects from below,

while the filtrate moves easily through the screen openings, causing a thickening of

the reject flow. Any pad build-up will be repeatedly destroyed by the rotor wings.

Dilution

Feed

Coarse reject

Reject

Accept P2

Accept S2

Accept T2

Pre-screen

stage

3 fine

screening

stages

Fig. 5.4 Left: Single-stage pressure screen (Metso TAP) [24].

Right: Multistage pressure screen (Metso MuST) [25].

The capacity and runnability of a screen, and also the screening efficiency to some

extent, can be controlled bymeans of pulsation. Today, several designs of foil-type pulsation

element are available. The aimis to obtain adequate pulsation by adjusting the

number of foils and their shape, width, clearance, incident angle, and tip speed. A

small difference between the peaks of positive and negative pulses is advantageous

for screening efficiency, but the negative pulses should be sufficiently strong to enable

the suction flow to remix flocks, fibers and contaminants resting on the edge of the

screen opening [26]. The general construction of rotors is shown in Fig. 5.5, with

a cross-sectional view on different pulsation elements of rotors in Fig. 5.6.