CHAPTER XXII
SODA RECOVERY
ONE of the most expensive items in a paper mill using esparto, straw, or
rags for its raw material is the caustic soda required for boiling. In small
mills, using rags or rags and wood, the spent liquor from the boilers, both in
quality and quantity, is not worth the expense of installing a recovery plant.
The disposal of the liquor is a continual source of trouble with the River
Pollution Authorities, yet it must be disposed of somehow, often at great
expense.
When a mill is boiling esparto and using 3 to 4 cwt. of caustic soda per
ton of grass, it becomes not so much a question of disposal as of recovery.
The principle of soda recovery is in itself very simple. The liquor from
the boilers, containing the soda combined with the non-cellulose constitu-
ents of the raw material, is evaporated to a thick liquor and incinerated.
The soda is thus converted into sodium carbonate, which is dissolved out from
the incinerated mass and brought back to the caustic condition by the action
of lime.
When the grass has been cooked, the steam is shut off and the vomiting
action ceases as the pressure falls. The drain cock to the spent liquor reservoir
is then opened and the liquor is discharged from the digester. A much better
result is obtained by allowing slight pressure in the digester to express the
liquid from the grass.
The cock is then shut and the boiler is filled, to above the level of the grass,
with hot water obtained from the condensation of steam during the boil*
Some steam is turned on to circulate the water through the pulpy mass.
This wash is also run to the tank. A second wash may be carried out in the
same way, but it is necessary to be careful here so that the liquor in the tank
is not too much diluted for economical recovery of soda. After this, wash-
ing water may be run to the drain until the effluent is reasonably clear. A
loss of soda occurs here of 2 to 5 per cent according to the efficiency of the
first and second washes. The second wash may be pumped to a separate
tank and used for the first wash of the next boil. Although this may reduce
the work of the evaporators to some extent it is bad practice, chiefly because
the colour of the grass is reduced, and any saving effected is nullified by the
extra pumping and tanks required.
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