56 MODERN PAPER-MAKING
process. At the upper end of the scale is the attempt to make the wood fibres
flexible with minimum loss of yield. Longer fibres can be expected by pro-
gressive removal of hemi-celluloses and lignin in conjunction with mechanical
separation of fibre bundles. High yields and cheap sources of wood are great
incentives to develop grades of semi-chemical pulps suitable for various boards
and papers*
CHEMICAL WOOD
The Sulphite Process.—'The sulphite process dates back to about 1870 and was
first used in America. The process consists in treating vegetable substances
containing fibres with sulphurous acid in water, and heating them in a closed
vessel or boiler under pressure. This action dissolves the incrusting matters
which are bound up with the fibres, and a fibrous product is left, suitable for the
manufacture of paper. During subsequent years several modifications of the
original process were introduced, the chief among them being those of Parting-
ton, Ritter-Kellner, and Mitscherlich. The preparation of the wood must be
carefully carried out; all rotten pieces, knots and blemishes have to be cut out
and the bark shaved off, usually by hand.
Some fresh knots may be left in during the boiling, which has little or no
effect upon them, and caught later in the screens. The cleaned and sorted logs
are passed to the chipper, which shaves them into chips about i by \ inch.
They are then carried on a travelling band and over a chip screen to the digester
house, where they fall through hoppers into the boilers.
The wood used for each boil should be of the same kind and preferably
of the same age. It is also an advantage that it should all contain the same
amount of moisture. Green or freshly-cut wood is most easily reduced by
the sulphite process. The wood in most general use is spruce or fir, though
other coniferous woods may be used.
The boilers or digesters are built of steel, but on account of the corrosive
action of the bisulphites and sulphurous acid a lining has to be added for pro-
tection. Innumerable experiments were tried, none of them very successful,
to utilise a lining of lead, but now acid-resisting tiles or bricks and coatings of
acid-proof cement are used.
The digesters are cylindrical in shape and for most processes they are vertical,
although horizontal boilers are used in some cases. They are usually very
large, 40 feet in height by 14 feet in diameter, or krger. They have manholes
at top and bottom for filling and emptying.
They are heated by direct steam or, as in the Mitscherlich process, by steam
coils made of lead, which are sometimes 2000 feet long. The digesters are