Bleach Plant Liquor Circulation

Andreas W. Krotscheck

7.10.1

Introduction

It is imperative for bleach plant operations to recirculate process liquors and to

minimize the consumption of fresh water. On the one hand, an increasing environmental

awareness calls for the responsible usage of water resources, yet on the

7.10 Bleach Plant Liquor Circulation 887

other hand there are strong economical arguments for liquor circulation, including

the reduced consumption of bleaching chemicals, energy and water itself, and

a lower hydraulic load to the effluent treatment facilities. Liquor circulation in the

bleach plant has certain limits, however, that are set: (a) on the process side, by

the accumulation of nonprocess elements or reaction products and by scaling;

and (b) on the equipment side, by the corrosion resistance of construction materials

or temperature sensitivity of machinery.

7.10.2

Intra-Stage Circulation and Circulation between Stages

It is possible to distinguish between two levels of bleaching liquor circulation, namely

intra-stage circulation and circulation between stages. Intra-stage liquor circulation

forms part of any modern bleach plant liquor management. As shown in Fig. 7.141,

filtrate coming fromthe washing equipment of a bleaching stage can be used to dilute

the pulp suspension in this very stage at various positions. Moreover, it can also be

used on the cleaning showers of the washing equipment itself. In some cases it may

be necessary to either heat or cool the dilution liquor in order to achieve a certain temperature

level in the dilute suspension. It may also be required to install a fiber filter

in a circulation line to protect the narrow shower nozzles from plugging.

Filtrate for dilution

Filtrate

For

Cleaning

Shower

Fig. 7.141 Generic intra-stage liquor circulation.

The circulation of liquors between bleaching stages is more delicate. Starting

with process matters, the advantages of liquor circulation include savings in energy

and chemicals. Clearly, the circulation of hot liquor to process steps where

heating is desirable and circulation of cold liquor to those steps where cooling is

needed, will save steam and cooling water. Likewise, the recirculation of filtrate

containing residual chemicals to the feed of a stage that uses such chemicals will

reduce the chemical charge required to reach a bleaching target.

In a given bleaching stage, both energy savings and chemical savings can be

usually achieved by using own filtrate as wash liquor on the preceding washer

(Fig. 7.142a). Depending on the bleaching sequence, other filtrates may, however,

at times be more appropriate than the own filtrate for such use (Fig. 7.142b). In

special cases, other filtrates may also be utilized for the dilution of incoming pulp,

reducing its consistency from the discharge consistency of the preceding washer

to the desired stage feed consistency (Fig. 7.142c).

888 7Pulp Bleaching

Pump,

Mixer,

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