Screening and Cleaning Applications

6.7.1

Selective Contaminant Removal

The selective removal of solid pulp impurities is by far the predominant application

of screening and cleaning in the production of chemical pulp. An overview

over the most common contaminants and their removal is provided below.

592 6 Pulp Screening, Cleaning, and Fractionation

Knots

Typically, knots represent the largest fraction of impurities in the pulp coming from

the digester. Knots originate from the dense sections of branches and heartwood, as

well as from oversized chips which have not been cooked down to their center. Knots

are rather large in size and of dark color. They can cause the failure of downstream

equipment in the pulp mill if they are not efficiently removed from the pulp.

Thus, knot removal (knotting) is normally carried out before washing. Knot separation

from the main stream of pulp is performed in a pressure screen. The separated

knots are then subjected to removal of good fibers in a secondary, atmospheric

screen. Both operations are governed by a barrier screening mechanism.

Shives

Shives are smaller impurities consisting of fiber bundles from incompletely

cooked wood. Their removal during screening is more difficult than that of knots.

Shives cause operational problems on the paper machine. In contrast to knots,

shives are mostly bleachable, but they consume higher amounts of bleaching chemicals

and may still remain of darker color than the bulk of the pulp after bleaching.

Shives should be removed before bleaching. Shive separation is carried out in a

system consisting of a number of pressure screens. Whether shive removal follows

barrier or probability screening depends on the aperture size of the screens.

As the use of very narrow slotted screens becomes common, shives tend to be

removed increasingly by the barrier principle.

Bark

Bark originating from incomplete debarking of the wood represents one of the

most challenging impurities. Bark is of dark color, has a similar density as wood,

and disintegrates easily.

There is normally no dedicated process for the removal of bark from pulp, but

the primary removal of bark should take place in the woodyard before chipping.

The remaining bark is removed from the pulp, together with other contaminants

during the course of screening and cleaning.