WOOD PULP 55
After the web of pulp has been concentrated it is pressed and wound up
on the top press roll until a sufficient thickness is obtained, when it is cut off,
laid in a pile about 2 feet long by 18 inches wide and pressed in hydraulic presses
for the removal of water. At this stage it contains about 50 per cent moisture,
and it is usually exported in this state, ready to be made into paper*
Mechanical wood pulp is used chiefly in the manufacture of newsprint,
of which it forms approximately 80 per cent; it is used also in the manufacture
of cheap printings, poster papers, boards and wrappings. It cannot, however,
be used alone, and must have a proportion of chemical wood pulp or other
stronger fibre mixed with it, in order to carry it over the machine-
Owing to the impure state of the cellulose contained in mechanical pulp,
papers made from it are gradually destroyed by oxidation, and they cannot
therefore be exposed to light, air, or damp atmosphere for any length of time
without being seriously affected.
The mechanical process is surprisingly successful in view of the direct
transition from wood to pulp by the wet grinding of pulp wood blocks on
abrasive stones. The grinder represents comparatively low capital cost, high
production, and moderate conversion cost where power is cheap. This is a
case where the best pulpwoods, such as spruce, have to be used for the cheapest
process, with serious loss of fibre length, but with preservation of bright colour
and cleanliness. The yield is over 90 per cent by weight of the dry wood
and over 95 per cent if screenings are taken into account. It would appear
that ground wood tonnages for world consumption will moderately increase
with the use of still higher percentages in newsprint and for further use in
board and low-grade printing papers.
To produce coarser fibres for insulation boards, hard-pressed board, and
other rapidly increasing substitutes for lumber, it is possible to disintegrate
wood waste by steam explosion, by dry shredding in hammer mills, by wet
refining, or in the case of wood blocks by crude grinding.
The semi-chemical processes have not yet reached great importance, but
much progress may be expected. Perhaps these pulps will not come into the
trading market in large tonnages, for the same reason that groundwood is
best adapted to direct conversion from slush pulp to the finished product. One
need only mention the wide range between mechanical pulp yield at over
90 per cent and chemical pulp yield below 50 per cent to realise the future
scope for medium cost pulps made by some combination of mechanical and
chemical treatments. Reasons for the deky in development have probably
been the low capacity and poor efficiency of refiners (compared with grinders
for pulpwood blocks), the tendency to lose bright colour of fibre in the presence
of weak chemicals, and the inherent difficulty of limiting the cost of a two-stage