BLEACHING SOLUTION 43
mixers, plenty of time is allowed for the settling of each charge. In fact, four
or more solutions can be made from each charge, the arrangements allowing
for endless variations of the procedure, besides keeping a constant and correct
solution in the store tank.
The three mixers, being regularly emptied to No. 4 tank along with the
sludge, are very easily kept clean. The store tank requires to be cleaned fre-
quently, as a residue of Krne will be found at the bottom, which becomes
very hard and difficult to remove. No. 4 tank is fitted with a manhole or door
close to the ground level, to facilitate the removal of the sludge.
Where only a small supply of bleach has to be prepared, one or two mixers
will be sufficient, as the sludge can be stirred several times and smaller charges
of powder used.
Careful supervision has to be exercised over this process to see that the
stock solution is kept at regular strength, and that it is not run off to the store
tank until it is quite clear.
Liquid Chlorine may be obtained in steel cylinders holding about a ton, or
in tank wagons ready for use. It may be used either as a bleaching agent or
as an auxiliary to bleaching powder. As we have already seen, bleaching
powder contains only about 35 per cent of chlorine, the remaining 65 per cent
is simply the 'carrier' and of no use. It has, in fact, to be washed away as use-
less sludge or lime mud, and it is difficult to get rid of in any case. About
75 per cent of this residue is lime, which can be combined again with chlorine
to form the same soluble bleaching agent as that which was extracted originally
from the bleaching powder.
By connecting a cylinder of liquid chlorine to the bleach mixer and allowing
the chlorine to run in while the agitation is going on, a very strong bleaching
solution is formed. The settling is much more rapid, and the chlorine will
have acted on the lime and combined with it, forming 'hypochlorite*, so that
the amount of the lime left as sludge, in the form of calcium carbonate, is
very small indeed, only about 25 per cent of the normal amount left, when
liquid chlorine is not used.
This means that the mixers need not be washed out so often, and also it
reduces the number of mixings or agitations always required in the first method
to extract the whole of the chlorine. This process, as will be easily seen, is
simply the rechlorination of the free or 'carrier* lime present in the bleaching
powder.
Most mills have now abandoned the use of bleaching powder entirely, and
are making all their bleach liquor from lime and liquid chlorine.
Various special forms of plant have been devised in which this process
can be carried on, but as a general rule paper-makers have preferred to adapt