32 MODERN PAPER-MAKING
exhausted, so that there is no waste. It entails, however, the provision of very
large draining tanks or chests, made of concrete and fitted with perforated
false bottoms. The liquor may be retained in contact with the stock for any
length of time? and then, by the opening of a valve the liquor is drained off
into a sump, and the rags are dug out and sent to the beater room.
One great disadvantage of this procedure is the fact that it exposes the
bleached rags to dust and dirt, which are always floating about, and it also
means that a great deal of labour has to be expended in digging them out
and conveying them to the beater room. To get over the difficulty of handling
the rags, they may be run, with the spent liquor, into hydraulic presses and
pressed into cakes ready to be taken up to the beaters or into a concentrator.
In this way they are more easily stored and kept free from dust.
The third method is to break and bleach the rags in a breaker situated
immediately above the beater. When the bleaching has been completed the
rags may be washed free of bleach liquor by putting down the drum washer
and running in fresh water for a quarter of an hour or so. When the beater
is empty the bleached stuff is run down straight into it, and so no time is lost,
no dirt comes in contact with the stuff, and no storage-room for half stuff is
required. This is satisfactory for good white rags, but to get the best colour
in low-grade rags the drainer method is by far the best.
The former method has the drawback that it is often necessary with a mixed
furnish of various grades of rags to break and bleach them together. It will
be obvious that this course is not always satisfactory, as such rags as linen,
canvas and cotton seconds do not require the same treatment in the breaker,
or the same amount of bleach. This trouble is usually overcome by breaking,
bleaching and beating the various components of the furnish separately, and
then mixing them in the machine chests. This is the most satisfactory method
from every point of view, but it requires sufficient beaters, and, above all,
very large machine-chest capacity to take the contents of a complete battery of
beaters at one time, It is also necessary that the breakers should be of much
greater capacity than the beaters, as the density of stock in each operation is
very different.
Unless the bleached rags are carefully washed before being emptied to the
beaters, a small amount of 'anti-chlor' must be added to die beater when
furnishing, when it is otherwise permissible, in order to free the stuff from the
last traces of chlorine. If this precaution is -not taken and much has been left
in the rags, endless trouble will be experienced from froth and variations in
the shade of paper.
The anti-chlors in general use are sodium sulphite and sodium hyposulphite,
and they depend for their efficiency on the percentage of sulphurous acid, as