360 MODERN PAPER-MAKING
agitator. A quantity of pure 'make-up' sodium carbonate is then added, with
the necessary amount of lime, to the causticiser, where it is dissolved by boiling
and agitation. The aim of the whole operation is to retain the maximum
amount of sodium carbonate, any losses being replaced at the causticiser.
The required amount of lime is then added to the causticiser, the action
being chemically designated so:
Na2CO3+CaO+H2O=CaCO3+2NaOH
Up to 45 per cent of lime is required, and the whole is subsequently boiled
for from i to 2 hours with direct steam and continuous agitation. Agitation
then ceases and solids, principally chalk and carbon, are allowed to settle.
'Strong' liquor may then be drawn from the top of the causticiser. It is then
recharged with water and agitated for a further hour. The solids are then
again allowed to settle as before and a weaker liquor is drawn off. These
two batches are then mixed and give a batch of boiling liquor at somewhere
about 14° Tw. The causticiser is again filled with water and agitated as before,
and when all solids have settled out the resulting liquor is drawn off and serves
as a 'water' for the next 'weak' liquor production.
The losses, which are inevitable, result from carrying off sodium carbonate
in the digested grass, in the sludge from the drum washer, if used, in the vapour
from the evaporating plant, by particle entrainment in the rotary furnace
flue gases, and incomplete recovery from the black ash, and sludge from the
causticiser pans.
The sludge left in the causticiser pan is a very difficult problem. It may
be drawn off and settled in ponds, the liquor from the surface being run away
and the sludge being dug out and deposited in some waste ground. It can be
readily understood that there may be many difficulties in this proceeding, as
it is not easy to find sufficient waste ground to dispose of the sludge from a
large mill.
It is possible, however, to dry this sludge and recondition it so that it may
be used as lime for agricultural purposes.
The recovery of from 70 to 80 per cent is possible economically, and
higher recoveries than this have been claimed. Although it is theoretically
possible to recover nearly 100 per cent of the original soda used, the expense
would be greater than the value of the recovered soda. Where there is no
easy method of disposing of sludge and extra washings of the grass owing to
the proximity of a preserved river, recovery has to be of the utmost possible
quantity in order to satisfy River Authorities. In this case extra expense is
of course inevitable and unavoidable.