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1116 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning

5.2Machines and Aggregates for Screening and Cleaning

Feed Feed Feed

Accept

Reject

semi-open closed open

f = Hydrofoil

Fig. 5.5 General construction forms of pressure screen rotors

(according to Niinimдki et al. [27]).

A Foils

B Bumps

C Radial vanes

D Tapered surface

Fig. 5.6 Rotor types with different pulsation elements (according to Bliss [28]).

Slotted screen plates have been found to have superior shives removal efficiency

compared to holed screen plates, but the capacity of the latter is better because of

a larger open area. Contoured screen plates have further increased the capacity of

slotted screens, and slotted screens manufactured from wedge wires (Fig. 5.7) provide

about 100% greater open surface area than screen plates with machine slots.

W S

P

W Wire width

S Slot width

P Contour height (profile height)

Fig. 5.7 Example of a wedge wire screen basket [29].

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

Continuous slotted screen Perforated cylinder

AFT MacroFlow™

Fig. 5.8 Pressure screen basket designs (AFT).

AFT EP Rotor™

Engineered Pulse™

AFT Gladiator™

HC Rotor

Fig. 5.9 Pressure screen rotor designs (AFT).

Some examples of industrial designs of screen baskets and rotors are shown in

Figs. 5.8 and 5.9.

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).

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5.2Machines and Aggregates for Screening and Cleaning

Fig. 5.10 Operation principle of a hydrocyclone.

1,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.

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