Consumption to a make-up in the amount of losses from the cycle. On the other

Hand, the organic material contained in the spent cooking liquor releases energy

For the generation of steam and electrical power when incinerated. A modern

Pulp mill can, in fact, be self-sufficient in steam and electrical power.

The main stations in the cooking chemical cycle are illustrated in Fig. 9.3. The

Digester plant is provided with fresh cooking chemicals, which are consumed during

The course of the pulping process. Spent cooking liquor contains, besides

Recovery

Chemicals, the organic material dissolved from wood. The spent liquor proceeds

To the evaporation plant, where it is concentrated to a level suitable for combustion.

The thick liquor goes on to the chemical recovery system, which comprises a

Recovery boiler and a number of installations for the preparation of fresh cooking

Liquor. The recovery boiler separates the inorganic cooking chemicals from the

Totality of spent liquor solids, and in parallel generates steam by combustion of

The organic matter in the spent liquor. The inorganics proceed to the cooking

Liquor preparation system, which in the kraft industry consists of the causticizing

And lime reburning areas. Fresh kraft cooking liquor is referred to as white liquor,

And spent kraft liquor is called black liquor.

COOKING

CHEMICAL

CYCLE

EVAPORATION

CHEMICAL

RECOVERY

COOKING /

WASHING

Fresh

Cooking liquor

(white liquor)

Spent

Cooking liquor

(black liquor)

Thick liquor

Fig. 9.3 The cooking chemical cycle.

Besides serving the purposes of chemical recovery and energy generation, the

Combustion units in the chemical recovery areas are used for the disposal of odorous

Vent gases from all areas of the pulp mill. From an environmental perspective,

these combustion units represent the major sources of a pulp mill’s emissions to

Air.

The following sections provide a brief overview over the main processes

Employed in evaporation and chemical recovery, with particular focus on the kraft

Process. Those readers requiring further detail are referred to the relevant (information

on recovery in the alkaline pulping processes e.g. Refs. [12–14]), and to

Ref. [15] for sulfite recovery.

9.2.2

Black Liquor Evaporation

Introduction

Spent cooking liquor coming from the digester or wash plant typically contains

Dry solids, the remainder being water. In order to recover the energy

Bound in the black liquor organics, it is necessary to remove most of the water

From the weak liquor. This is done by evaporation, and this in turn raises the dry

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