The equilibrium of the slaking reaction is far on the product side of Eq. (12),

and slaking is completed within 10–30 min. In contrast, the equilibrium of the

Causticizing reaction in a typical kraft pulp mill would be reached at a causticizing

efficiency of about 85–90% (Fig. 9.11). The equilibrium conversion rate depends

Mainly on total alkali, sulfidity, lime quality, and temperature. A higher TTA and

Sulfidity reduce the equilibrium causticizing efficiency through product inhibition.

As the retention time proceeds, the causticizing reaction is increasingly limited

By the diffusion of reactants and reaction products through the increasingly

Thicker layer of CaCO3 around the hydroxide core of the particle. The equilibrium

Efficiency can be reached only with an excess of lime and at very long retention

Times.

Actual mill operations deal with time restrictions, and must avoid over-liming

In order to maintain good filterability of the white liquor. As a consequence, the

average achievable causticizing efficiency on mill scale is 3–10% lower than the

Equilibrium efficiency. Typical total retention times provided in the causticizers

Are around 2.5 h.

Chemical Recovery Processes 987

70% 75% 80% 85% 90% 95% 100%

Total titrable alkali (TTA), g/L as NaOH

Causticising efficiency

Total titrable alkali (TTA), g/L as Na2O

Typical operation window

Sulphidity

Sulphidity

Fig. 9.11 Equilibrium causticizing efficiencies at 0% and 30%

sulfidity [14] and typical operating window for kraft mill causticizing

Systems. Sulfidity in this diagram is defined as

NaOH/(Na2S + NaOH + Na2CO3).

White Liquor Preparation Processes and Equipment

The preparation of white liquor begins with smelt dissolving. Weak wash and

Smelt form the green liquor, a solution of mainly sodium carbonate and sodium

Sulfide. The green liquor carries some unburned carbon and insoluble inert material

From the smelt, which are detrimental to the downstream recovery and pulping

Processes if not removed, together with lime mud particles. While the removal

of these so-called “dregs” was traditionally carried out by sedimentation, the

requirements of today’s increasingly closed mills are best met with green liquor

Filtration. Since filters retain much smaller particles than clarifiers, the levels of

Insoluble metal salts are kept low. Several types of filters with and without lime

Mud filter aid are in use, such as candle filters, cassette filters, crossflow filters or

Disk filters.

The dregs separated from the green liquor are subjected to washing for recovery

Of valuable cooking chemicals. Dregs washers are typically rotary drum filters

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