Overview. First, we will examine the phenomena which influence pulp washing operations

First, we will examine the phenomena which influence pulp washing operations

from a theoretical perspective. Due to the complexity of the involved mechanisms

and a certain lack of practically applicable mathematical relationships, this theoretical

discussion will be mostly qualitative in nature.

In the basic case of washing, we are dealing with a liquor-soaked network of

pulp fibers – often called the “pulp mat” – which is charged with wash liquor. The

pulp mat forms on the filter medium, a perforated device such as a wire or a

screen, which holds back the pulp but allows the extraction of filtrate from the

pulp mat (Fig. 5.1).

Pulp mat

Wash liquor

Filtrate

Wire / screen

Liquor trapped

between fibers

and in fiber voids

Free-flowing liquor

around fibers

Pulp fibers

Fig. 5.1 Simplified illustration of the pulp

washing operation. Wash liquor is added to the

pulp mat which is retained on the filter medium

(wire or screen), and filtrate is extracted

through the filter medium. Right side:

Simplified illustration of the pulp suspension,

with free-flowing liquor around the fibers and

immobile liquor trapped between the fibers

and in the fiber voids.

For the purpose of facilitating the understanding of the involved processes, the

phases participating in the pulp suspension can be divided into three fractions:

(a) a free-flowing liquid phase around the fibers;(b) an immobile liquid phase

locked in between the fibers and within the fiber voids;and (c) the fiber solid [1].

Specific considerations are applicable for each of the three fractions. On one

hand, the free-flowing liquid together with the substances dissolved therein can

be drained, displaced by another liquid, or pressed from the fiber network.

On the other hand, the immobilized liquid inside and in between the pulp fibers is

less accessible. Removal of this liquid is limited to pressing. Otherwise, the mass

transfer fromthe immobile liquid to the free-flowing liquid is controlled by diffusion.

As for the fiber solids, a part of the dissolved substances may be bound to the

fibers by sorption and may as such be barely removable at all.

512 5 Pulp Washing

5.2.2