7o MODERN PAPER-MAKING
practically free from resins by reason of the alkaline cooking. Unbleached
sulphite is the chief problem, because the acid cooking makes the pitch more
sticky. Great precautions are taken to avoid harmful character of pitch and
to lower the percentage in the pulp. The bleaching process greatly reduces
the pitch content. For chemical uses the bleached sulphite is treated with
alkali and sometimes with saponifying agents to remove practically all
the resins.
Small quantities of foreign matter have sometimes to be controlled for
special uses. Iron and copper, either in specks or in diffused form,- must be
kept to low figures in bleached pulps for the manufacture of photographic
paper, genuine vegetable parchment, and other special grades. Chemical
pulp mills are using more and more glazed tile, stainless steel, rubber, and other
resistant surfaces to avoid impurities. Sulphur residues must be reduced below
specified figures in sulphite and sulphate pulps for anti-tarnish paper and similar
products, but groundwood is naturally pure in this respect.
The reddening of unbleached sulphite is due to oxidation of colouring
bodies, and the tendency towards reddening may be judged by testing with
hydrogen peroxide. Reddening is usually greatest with strong sulphites and
least with easy bleaching grades, but the wood species and the pulping conditions
also have a bearing. Reducing agents, such as sodium thiosulphate, have a
protective action.
REDUCTION OF ESPARTO GRASS TO HALF STUFF
The bales being opened out, the grass is put through a willow or duster
similar to a threshing machine, which breaks up the bunches and loosens and
separates the sand, dust, etc., from the blades. From the duster a conveyor
carries it to the boiler. The boilers are usually of the stationary vomiting
type. A revolving boiler causes too much loss of fibre through friction, and
packs the grass into hard heaps very difficult to deal with. A usual size holds
about 2£ or 5 tons of dry grass. Boiling is conducted with about 14 to 15 per
cent caustic soda, from 3 to 4 hours, with 40 to 60 Ik steam pressure, depend-
ing upon the quality of the grass and the capacity of the boiling plant to keep
up with the demands of the irdll.
The caustic liquor is run in .with the grass, and some steam turned on in
order to soften it and get a heavier charge into the boiler. On the completion
of the boil, the steam is blown off and the used liquor is run to the recovery
plant. Further washes with hot water to remove the last of the liquor are
necessary, and these may also be run to the recovery plant or liquor tanks.
The grass is then dug out and filled into trucks, or it may be more conveniently