Principles of Washing

In this section, we will review the different possibilities of bringing the pulp and

washing medium into contact. Pulp washing is accomplished either with clean

water or, more frequently, with a washing liquid of a certain composition. Here,

5 Pulp Washing

the general term “wash liquor” will be used to describe both clean water and washing

liquid.

5.3.1

Dilution/Extraction Washing

The simplest method of washing is by repeated dilution and extraction (Fig. 5.11).

In the first step, the pulp feed is mixed with the wash liquor (1), after which the

filtrate is extracted and the pulp discharged (2).

Pulp feed Pulp discharge

Filtrate

Wash liquor

1 2

Fig. 5.11 The principle of dilution/extraction washing.

Dilution/extraction washing will not be effective unless it is repeated many times.

In theory, an infinite number of dilution/extraction washing stages is needed to

bring the concentrations in the pulp discharge to their levels in the wash liquor.

The efficiency of this operation is generally low, and depends primarily on the

consistencies to which the pulp is diluted and thickened. It also depends on the

extent to which solute is sorbed on the fibers and the time required for solute to

diffuse out of the fibers.

In a modern pulp mill, there is no room for dilution/extraction as a separate

washing process. Nevertheless, dilution/extraction phenomena occur when process

reasons require a dilution, whether for fiber separation during screening, for

even fiber distribution in the mat formation zone of a washer, or for homogeneous

mixing of chemicals.

5.3.2