Multi-Stage Washing

Frequently, one washing stage alone is insufficient to carry out the required washing.

In such a case, multi-stage washing must be performed either on a number

of single washers in series, or on one piece of multi-stage washing equipment.

The number of stages depends mainly on the necessary washing efficiency, on the

pulp furnish, applied equipment and on the liquor management.

In a multi-stage system, the maximum solute removal could be achieved if the

pulp were washed in each stage with fresh water. However, this method of multistage

washing results in a huge overall amount of very dilute filtrate, and is therefore

not acceptable in practice.

Modern multi-stage wash plants utilize the countercurrent principle, where the

wash medium flows countercurrent to the pulp flow. The pulp is contacted with

the cleanest available wash liquor before it leaves the last washing stage. The filtrate

from the last stage is sent back in the opposite direction of the pulp flow to

serve as wash liquor on the next-to-last stage, and so on, until it reaches the first

washing stage (Fig. 5.14). The filtrate from the first washing stage has the highest

concentrations and is ready for processing elsewhere in the mill, for example, in

the evaporation plant.

5.3 Principles of Washing

Washing

stage 1

Pulp feed Pulp discharge

Filtrate

Wash liquor

Washing

stage 2

Washing

stage 3

Washing

stage n

Fig. 5.14 The principle of countercurrent washing in a washing system incorporating n stages.

Besides the advantage of delivering a limited amount of filtrate at high concentrations,

countercurrent washing also features a reasonable energy efficiency, as

the amount of filtrate also limits the thermal energy leaving the system with this

filtrate.

At a given number of washing stages, the efficiency of multi-stage washing is

influenced by the method of pulp transport between stages. Intermediate mixing

of the pulp slurry, for example due to pump transfer to the next stage, reduces the

washing efficiency compared to the unaltered advancement of the pulp mat. In

the latter case, a concentration gradient over the height of the mat is maintained

between the stages, which eases washing in the second stage (Fig. 5.15).

Pulp feed Pulp discharge

Wash liquor

Pulp feed Pulp discharge

Wash liquor

Filtrate

Filtrate

Fig. 5.15 Two-stage countercurrent washing with and without intermediate mixing.

5 Pulp Washing

5.3.5

Дата добавления: 2015-10-21