350 MODERN PAPER-MAKING
THE RECOVERY AND RE-USE OF WASTE WATERS, FIBROUS MATERIAL
AND LOADING
In almost all mills a large quantity of fibre and clay may be recovered
from the back waters of the machines. Generally speaking, the lower the
grade of paper being made the greater is the amount of stuff which can be
recovered.
Until quite recently it was common practice for the waste waters con-
taining size, colour, loading and fibres to be run to the drain, river or other
convenient place, and thus much valuable material was lost and much pollution
of rivers caused.
Two factors were chiefly responsible for the curtailment of these losses, one
being die increased competition in the trade and consequent need for economical
production, the second the tightening up of bye-laws concerning pollution by
river conservators and fishery boards.
The yield of material in a rag mill may be taken to be from about 60 to
80 per cent, so that from 100 tons of rags anything from 20 to 40 tons are
lost between the rag loft and the machine reel. A good deal of this loss is
represented by unsuitable material thrown out, and also by the removal of
dust, loadings, starch, grease and ligneous matter during the boiling and bleach-
ing operations.
Apart from this, however, there is a serious loss of small fibres during the
washing and draining of the stuff, and again at the machine, where a great deal
of small fibrous material is washed away through the meshes of the machine
wire. There is also a loss at the auxiliary strainer, due to hanks and knots of
fibres which get caught here, which are so mixed with dirt, rubber, etc., as
to be unfit for further use.
Stuff is also lost which runs down the wire when starting up, or if a break
occurs there, and other small losses occur when starting and stopping by stuff
going down the pit between the wet felt and second press, and between the
second press and cylinder. These latter losses are very small, and may be
prevented by the provision of clean wooden boxes to catch the stuff, provided
that'the stuff is carefully removed at frequent intervals and returned to the
beaters in a clean state.
Where the mill makes nothing but fine qualities it will generally be found
impossible to use again any of the fibres which may be recovered from the
washing waters and the auxiliary strainer. The usual practice in dealing with
such waste is to run it to tanks where the fibrous material may settle down, and
the clear water is then run off to the river or drain. The tanks are periodically
cleaned out and the stuff is thrown away as useless refuse.