WOOD PULP

59

grain of the wood, whereas in the soda processes the liquor is able to penetrate
the chips in all directions.

After the pulping processes come the usual operations of riffling to setde out
heavy impurities such as grit, and screening to separate knots, fibre bundles,
and bark specks as carefully as possible. Kollerganging, rod-milling, or other
light brushing treatment is often applied to kraft pulps. The coarser fraction

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FIG. 15.—TRANSPORTING LOGS INTO THE MILL
of mechanical pulp will benefit in strength and uniformity if present attempts
to brush out the fibre bundles are successful*
Special mention should be made of fibre selection. Chemical pulps are
sometimes treated to separate the longer, stronger fibres of higher purity. In
the case of sulphite pulp, the removal of most of the pitch with the medullary
ray fibres is also a feature.
The drying of chemical pulp over cylinders has been superseded in a number
of mills by vacuum drying at low temperature in the absence of oxidising
atmosphere, or by hot-air drying at comparatively low temperature, with a
slight gain in ease of beating, brightness, and strength of pulp at the paper
mills. Care in the baling of pulp for shipment has reached a high standard.