Introduction. After cooking, the pulp contains undesirable matter of different provenance

After cooking, the pulp contains undesirable matter of different provenance. On

the one hand, there are wood components other than separate pulp fibers, such as

dense sections from branches and heartwood, bark, or uncooked chips. On the

other hand, certain non-wood contaminants are carried along with the wood chips

or enter the pulp mill processes by other routes. These contaminants may include

stones and sand, as well as metal or plastic debris.

The objective of screening and cleaning is to remove these solid impurities

from the pulp for the purposes of protecting downstream equipment, saving

bleaching chemicals and last – but not least – to obtain a clean final product.

Screening and cleaning equipment can also be used for pulp fractionation. During

fractionation, the totality of the pulp is divided into usable portions of different

properties – for example, fiber length or fiber wall thickness – which are separately

handled downstream in the process. In chemical pulping, fractionation by

screens has traditionally been employed in high-kappa applications. Fractionation

for low-kappa pulps has gained industrial significance only recently.

Screening operations can be located in both the unbleached and bleached sections

of the fiberline. Centrifugal cleaning is usually employed for the bleached

pulp, except for cleaning for sand removal which is also used in unbleached

screening systems. Demands on screening operations have increased since environmentally

friendly bleaching sequences have been substituted for chlorine as a

bleaching agent. Shives which previously were bleachable by chlorine must now

be efficiently removed.

Screening with narrow slots is emerging as a challenge to centrifugal cleaning

as the process of choice for contaminant removal from bleached pulp. With regard

to fractionation, pressure screens and centrifugal cleaners cover different separation

tasks. While screens fractionate mainly by fiber length and fiber flexibility,

cleaners fractionate mostly by wall thickness, coarseness, and fibrillation.

Traditionally, screening and cleaning have been targeted at the removal of solid

impurities from pulp fibers, and for most applications this still holds true. Hence,

Handbook of Pulp. Edited by Herbert Sixta

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-30999-3

©2006 WILEY-VCHVerlag GmbH&Co.

Handbook of Pulp

Edited by Herbert Sixta

much of the screening and cleaning nomenclature used in the industry is related

to this situation. In this chapter, however, we will at times attempt to use a more

general approach to the separation phenomena encountered in screening and

cleaning, which also accounts for the increasing importance of these process steps

in fractionation.

Pulp screening can be described as solid-solid separation using a screen in a liquid

environment. The larger particles are retained on the screen, while the smaller

ones selectively pass through the narrow screen apertures. The mechanism of

screening is separation by size. For example, large pulp fiber bundles are retained

on the screen while small isolated fibers pass through.

Pulp cleaning is characterized by solid-solid separation using centrifugal and

centripetal forces in the gravity field of a hydrocyclone. Depending on the separation

task, the design of the hydrocyclone is customized so that either heavy-weight

or light-weight particles are selectively separated from the main stream. The

mechanism of cleaning is separation by specific weight. For example, heavyweight

sand can be separated from the light pulp fibers.

For all separation applications it is essential that the solid components can

migrate freely within the pulp suspension. Unfortunately, the particle–particle interaction

of pulp fibers results in flocculation even at very low consistencies. Flocs,

which are stochastic clumps of fibers, constrain the free motion of both undesirable

matter and good fibers. Hence, flocs must be disintegrated in order for the

separation to be efficient. Only when particles do not interfere one with another

does the actual size of a single particle determine whether it passes the screen

aperture, or not. Likewise, the weight of a single particle determines which direction

it moves in the gravity field of the hydrocyclone, but only if the particles do

not interact.

The necessary deflocculation of fibers leads to a physical limitation with regard

to pulp consistency in the feed to the separator. The high shear forces needed to

disintegrate flocs at the consistency of pulp screening are generated mechanically,

by the rotor in a pressure screen and by the vibrations of a vibratory screen. As

hydrocyclones have no moving parts and require laminar flow for their correct

operation, their application is restricted to lower consistencies.

Previously, screening and cleaning equipment utilized atmospheric feed and/or

discharge. However, today’s pressurized systems have many advantages over their

atmospheric counterparts, including a higher operating consistency, an elevated

operating temperature, a greater capacity, a lack of air entrainment, and a better

separation efficiency.

In this chapter we will first examine, from a theoretical perspective, the phenomena

and parameters that influence separation in screening and cleaning

equipment. This will facilitate an understanding of the processes involved. Subsequently,

screening and cleaning applications and systems design will be discussed,

before focusing on separation equipment.

562 6 Pulp Screening, Cleaning, and Fractionation

6.2

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