Separation according to material density is carried out using hydrocyclones

(Tab. 5.1 and Fig. 5.10). In almost all of these apparatus, the pulp suspension is

Fed tangentially by pump pressure into a cylindrical or conical pipe and moves as

A spiral downwards at the outer wall of the pipe. Heavy particles are found outside

Of the downward stream moving slowly through an opening at the lower end of

The pipe into a dirt collector. The main stream is turned up above this opening

And rises in the middle of the pipe to the outlet. Depending on the design, the

Pressure drop in this hydrocyclones is from 60 to 300 kPa (0.6 to 3 bar).

Machines and Aggregates for Screening and Cleaning

Fig. 5.10 Operation principle of a hydrocyclone.

Pulp suspension inlet; 2,accepts; 3, rejects.

Tab. 5.1 Parameters of centrifugal separation principle for different cleaner types.

Parameter Centricleaner Low-consistency cleaner

Pulp consistency [%] < 1 < 1

Pressure drop, [kPa (bar)] 300 (3) 160 (1.6)

Acceleration 550 g 150 g

Inlet flow rate [m s–1] 20 12

Fiber loss [%] 2 Practically none

Specific energy consumption [kWh t–1] 0.4–0.5 0.08

It is impossible to design a screen for mechanical pulps that is able to separate

Particles depending on the screen size absolutely. In practice, there is always only

A certain amount of shives separated, and this depends mainly on the overflow

Rate. In addition to the efficiency grades of the single separation aggregates, a

Combination of all single screens estimates the efficiency and economy of the

Screening stage.

Some schematic examples of modern screening concepts are illustrated in

Figs. 5.11–5.13.

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

Feed Accept

P1 / hole P2 / slot

S2 / slot

To reject treatment

P1 Primary screen 1, hole basket

P2 Primary screen 2, slot basket

S2 Secondary screen 2, slot basket

Fig. 5.11 Modern main line screening.

Feed Accept

Reject (to reject treatment)

P1 / slot

S1 / slot

T1 / slot

P1 Primary screen 1, slot basket

S1 Secondary screen 1, slot basket

T1 Tertiary screen 1, slot basket

Fig. 5.12 Thermomechanical pulp (TMP) screening.

Feed Accept

Back to reject treatment

R1 / hole R2 / slot

R1 Reject screen 1, hole basket

R2 Reject screen 2, slot basket

Fig. 5.13 Screening concept for refined groundwood rejects.

Reject Treatment and Heat Recovery

5.3

Reject Treatment and Heat Recovery

The separated coarse reject is treated in special reject disc refiners. In grinding, a

High-consistency refining of the reject is also useful. The technological scheme of

A high-consistency reject refining stage is shown in Fig. 5.14. The pulp arrives

from the reject bin with 2.5% consistency and is thickened up to 25–30%.

Fig. 5.14 Technological scheme of a high-consistency reject refining stage.

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