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washed, and finally discharged from the drum at 35–50% dry solids by a slowly
Advancing scraper together with a thin layer of precoat. The consumption of lime
Mud for the precoat amounts to at least the same quantity as the dregs. This consumption
Is, however, not considered a loss because some lime mud must be
Sluiced from the lime cycle anyway for process reasons. Otherwise, nonprocess
Elements would accumulate in the lime cycle to problematical levels.
Clear green liquor coming from clarification or filtration proceeds to slaking.
An example of a slaker is shown in Fig. 9.12. Lime mud and green liquor enter
The equipment from the top of the cylindrical slaker bowl and are intensely mixed
Recovery
By an impeller. Not only the slaking reaction, but also a major part of causticizing
Occurs in the slaker. The slurry flows from the slaker to the classifier section, from
where it overflows to the causticizers. Grits – that is, heavy insoluble particles
such as sand and overburned lime – settle in the classifier. These are transported
By an inclined screw conveyor through a washing zone, and leave the cooking
Chemical cycle for landfill, together with dregs.
Fig. 9.12 A slaker [16].
The slurry from the slaker enters the first of typically three causticizers, each of
Which is divided into two or three compartments (Fig. 9.13). The slurry flows
From one unit to the next by gravity. Minimum backmixing between compartments
Ensures that the causticizing efficiency advances to a maximum. Agitation
In slakers and causticizers needs special attention in order to avoid particle disintegration,
Since small lime mud particles reduce the white liquor filterability.
Fig. 9.13 A causticizer train [17].
After causticizing, the lime mud is removed from the slurry by pressure disk
Filters or candle (pressure tube) filters. The use of clarifiers for that purpose is fading
Out. A pressure disk filter, where the slurry from the last causticizer enters the
Filter vessel at the bottom of the horizontal shell, is shown in Fig. 9.14. White
Liquor is pushed by gas pressure through the precoat filter medium on the rotat-
Chemical Recovery Processes 989
Ing disks into the center shaft, and flows to the filtrate separator. There, the white
Liquor and gas are separated. While the white liquor proceeds to storage, a fan
Blows back the gas from the top of the filtrate separator to the shell of the disk
Filter to provide the driving force for filtration. Lime mud accumulates on the filter
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The equilibrium of the slaking reaction is far on the product side of Eq. (12), | | | Medium as it rotates submerged in slurry, is then washed and continuously |