48 MODERN PAPER-MAKING
The batter}- shown in the illustration consists of a breaker fitted with two
ordinary drum washers, and is capable of dealing with the stock for 60 tons
of paper per week. The stuff is pumped from the breaker to the first tower,
which is fitted with two discharge pipes, one into the second tower, and one
back into the first tower. The first or fourth towers have this arrangement
of pipes and also spreader hoods for thoroughly mixing the pulp. The last
tower is fitted with a concentrator, by means of which the last traces of bleach
liquor are removed, after washing in the last tower, and the thickened pulp is
run or pumped direct to the presse-pdte chest.
All the pumps attached to the towers may be driven from the same shaft.
This arrangement of bleaching towers, worked in conjunction with a battery
of Taylor or Tower beaters, forms an excellent equipment for dealing with
esparto and wood-pulp stock.
The pulp is mechanically handled throughout, and is enclosed and away
from dirt and dust the whole time.
The first tower is used for adding the bleach and mixing, and the last tower
for washing. The capacity of the plant depends on the number of towers
installed, but with each tower holding 2 tons dry weight, and delivering this
quantity every.4 hours it will be seen that the capacity of a five-tower plant
will be approximately 60 tons per week of finished paper.
Bleaching Wood Pulp.—For bleaching wood pulp, an ordinary Hollander
type potcher of large capacity is generally used. This may be made of cast
iron or concrete lined with glazed tiles to assist circulation, and is fitted with
a large cast-iron roll with projecting bars.
Steam is always used to heat the stock after the bleach has been added,
but the temperature should not be allowed to exceed about 80° F., or damage
will be done to the fibres and the colour of the half stuff will deteriorate if
it is allowed to lie in drainers.
The action of the bleach is very rapid at first, so that a quick and thorough
mixing and circulation of the stock is necessary. When heat is used, the bleaching
is accomplished in about 3 or 4 hours, but if drains are provided the stuff may
be emptied after about the first hour or so, and the bleaching completed while
the pulp is at rest. When, however, the bleached stuff is to be run straight
into the beaters, at least 3 hours has to be allowed, with from 6 to 10 per cent
of bleaching powder, calculated on the dry weight of pulp. The amount
required, of course, depends on how easily the pulp bleaches, the colour
required and the time available.
In this latter method, when the pulp is run straight to the beaters, the bleach
residues must be removed by the use of a drum washer and fresh water, and
it will usually be necessary to add a little anti-chlor until no trace of bleach is