Feed and Discharge Consistencies

The feed consistency has a major influence on the electrical energy consumption

and on the design of equipment. On the one hand, a low feed consistency means

that large volumes of filtrate are pumped to dilution points before the washer, and

consequently a lot of electrical energy required for pumping.

On the other hand, the feed consistency determines the amount of liquor that

must be removed from the pulp suspension in the dewatering zone of the washing

equipment. The lower the consistency, the larger will be the dewatering zone

of the washing equipment and also the filtrate tank. For a given washer, the feed

consistency has a considerable influence on the capacity (Fig. 5.18).

20%

40%

60%

80%

100%

0.6 1.0 1.4 1.8 2.2

Relative washer capacity

Pulp feed consistency [%]

Fig. 5.18 Effect of pulp feed consistency on the capacity of

a pressure washer, as calculated from Eq. (15).

If the pulp discharge from the washer is at medium consistency (12–14%), the

quantity of liquor leaving the equipment with the pulp exceeds the amount of

pulp by a factor of 6–7. At the high discharge consistency of a press (30–35%),

this factor is reduced to just two. Evidently, the liquor accompanying the pulp in

the press discharge may be three times as concentrated as the liquor coming with

the medium consistency discharge to still have the same per-ton washing loss.

High consistency discharge allows for a distinct separation between process

stages. If a washer discharges pulp at high consistency and the next process steps

occurs at medium or low consistency, the necessary dilution offers a good possibility

for temperature or pH adjustment, or for chemical recycling. This usually

leads to savings in chemicals, water, and energy.

5.4.4

PH

The pH encountered on a pulp washer is normally a function of the preceding

process step and the origin of the wash liquor. In most cases, it is not feasible to

use the pH as a control parameter for washing. There are cases, however, when an

adjustment of the pH is desirable and beneficial.

First, there may be a pH change on the washing equipment, for example, when

alkaline feed stock is washed with acidic wash liquor. Occasionally, such a constellation

provokes scaling on the surfaces of the washer due to precipitation of inorganic

or organic compounds. If this is the case, process measures must be taken

to move the unfavorable pH region away from the washer.

Second, pulp washing operations typically are more challenging under alkaline

than under acidic conditions. On the one hand, fiber swelling under alkaline conditions

is made responsible for somewhat reduced drainage rates. On the other

5.4 Washing Parameters 533

hand, soaps act as surfactants and induce foaming issues as well as increased

entrainment of air in the filtrate. Under mill conditions, the latter effect seems to

be a more critical influence on the washing performance [9].

Improved drainage can be observed as the pH of a highly alkaline pulp is lowered.

It is suspected that the reason for this behavior lies in sodium phenolate

soaps being converted to their protonated form, in which they no longer function

as surfactants. The drainage rate begins to improve as the pH is reduced below

11, and levels out around pH 9,5. Further reduction beyond this point brings no

more washing performance improvement. Instead, there is a quality risk of reprecipitation

of dissolved lignin on the fibers.

Carbon dioxide and sulfuric acid have been found to be equally effective for

improving drainage and reducing foam [9], but any other inorganic acid should

do as well.

For any given piece of washing equipment, improved drainage means a higher

outlet consistency and consequently a higher washing efficiency. With regard to

foaming, there is a trade-off between acidification and the reduced use of defoamer.

5.4.5

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