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

with a lime mud precoat. As the filter drum rotates, the dregs are dewatered,

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

988 9 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-

9.2 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

medium as it rotates submerged in slurry, is then washed and continuously

scraped from the surface of the disc at 60–70% dry solids. The lime mud is then

discharged through a number of chutes into the mud mix tank. The washing step

leads to a minor dilution of the white liquor, but reduces the requirements of

downstream lime mud washing.

Fig. 9.14 A pressure disk filter [18].

The examples of equipment solutions described above are what will most likely be

found in a new mill. Existing causticizing plants are likely to appear quite different, as

they may have seen certain pieces of equipment taken into different service over time

as causticizing capacity increased. Equipment with potential application in changed

positions includes rotary drum filter for the washing of dregs or lime mud; candle

filters for white liquor filtration or lime mud washing; and sedimentation clarifiers

for clarification of green liquor or white liquor, or for lime mud washing.