BLEACHING 49

left. Unless this is done, endless trouble will be experienced in keeping the
colour of the paper from varying in shade.

In the former method, where the pulp is allowed to lie in the drainer, it should
not be left for very long, otherwise it will begin to go back in colour, and that
part of it which is exposed to the atmosphere will turn yellow. The bottom
portion nearest the floor of the drainer will also discolour badly, and will retain
a good deal of water and bleach residues, unless the perforations in the false
bottom of the drainer are kept clean.

[Messrs. Jamts Bertram and Sons Ltd.
FIG. ii.—THREE-DRUM WASHING AND CONCENTRATING MACHINE, FOR WOOEK-PULP,
RAGS OR ESPARTO GRASS
Bleaching should always be carried out with the stock at as thick a con-
sistency as possible. The density, of course, will depend on the type of machine
in which the bleaching is carried out, but even if it is in an ordinary Hollander,
the stock should be as thick as possible.
A cheap, speedy, and continuous method of drying bleached pulps to about
70 per cent moisture or less is sorely needed by many mills to-day. It is true
that there are several Rotary Vacuum Filters on the market, but these are
troublesome and inefficient in many ways. Some of them work quite well
on wood. On rag stuff and esparto there is the presse-pate which does well in
some mills and is continuous. There are hydraulic presses and hydro-extractors,
but these are both slow and intermittent. The continuous screw press has not,
unfortunately, been brought to a sufficient satisfactory state of perfection to
be suitable for all types of fibres. The multiple drum concentrator (Fig. u)
is used in many mills, and this is satisfactory where the stuff can be taken straight
to the beater.