58 MODERN PAPER-MAKING
or in relation to the purification of the final bleached fibre. The ammonium
bisulphite process in Norway aids the recovery of valuable by-products from
the spent liquor, and also has a bearing on the pulping of resinous woods.
The sodite process in America employs alkaline sodium sulphite and yields
a more distinctive grade of bleached pulp.
Soda Process.—In this process the chipped wood is boiled with caustic soda
under pressure. No attempt is made to drill out knots or rotten parts of logs,
as the soda process is so drastic as to eliminate these impurities. The resulting
pulp is of a brownish shade, and does not bleach to .so good a colour as sulphite
pulp. The fibre is long, strong, and bulky.
Pulp from poplar, produced by this process, is very soft and absorbent.
It is used in certain papers as a substitute for esparto and for cheap blottings.
This was the first chemical method of boiling pulp. The caustic soda
treatment is satisfactory for aspen and harder deciduous woods to produce
bleached filler fibres. Actually these mills are confined to America and are
not expected to increase materially, because a greater number of sulphite and
sulphate mills in the various forest countries can handle any hardwoods within
reach. Yields are lower than by the sulphate process, which has the protective
action of sodium sulphide.
Soda Sulphate or Sulphate Process.—This is simply a variation of the soda
process. In the process of soda recovery, sodium sulphate is added to make
up the loss of soda, and the liquor comprises a mixture of caustic soda, sodium
sulphide, and sodium sulphate. This process has extended rapidly during recent
years. The alkaline nature of the cooking liquor (caustic soda and sodium
sulphide) makes it of universal application to all species of pulpwoods, including
the very resinous. The trend is naturally towards the pines which grow both
in northern and southern countries and which are subject to less competition
than spruce. This means that sulphate pulp mills have the widest distribution,
sometimes an advantage in proximity to markets. The special feature of the
sulphate process is the ease of preserving the inherent strength of the wood
fibres. The alkali resolves the bark and knot specks, and the shives to some
extent are obscured by the brown colour of the pulp. The yields are -close
to those of unbleached sulphites, but tend to extend lower for easy bleaching
sulphates. Total consumption of sulphate pulp has now reached half the
sulphite tonnage, and promises to account for a larger proportion in future,
perhaps chiefly for boards and in terms of bleached sulphate grades for many
papers.
In-connection with chemical pulp cooking processes there is a distinction
to be noted between sulphite and soda in the mechanism of penetration. -In
the acid process the cooking liquor enters the chips almost entirely along the