60 MODERN PAPER-MAKING
Bleaching and other chemical treatments of pulp may be looked on as
processes of secondary digestion. Great advances have been made in recent
years with the better knowledge of cellulose chemistry.
The bleaching of mechanical pulp is based on the reducing action of bisul-
phites or hyposulphites, which brighten the yellowish colouring substances
in groundwood without affecting strength or yield to any extent.
Chemical pulp bleaching has undergone revolutionary changes during the
past ten years, based on laboratory methods known much earlier. The use
of over half the total chlorine in the first stage for direct chlorination of the
unbleached pulp at low temperature and consistency with thorough mixing,
permits rapid formation of lignin products without appreciable change of the
cellulose itself or reduction of shives and bark specks. Neutralising the hydro-
chloric acid formed in this reaction brings the chlorinated products into solu-
tion, and washing avoids the subsequent bleach consumption by coloured
liquor left in a single-stage process. Second stage bleaching with calcium or
sodium hypochlorite at low temperature and high density of pulp favours the
oxidation of colouring matter and resolution of bark specks and shives under
the mildest conditions. After again washing the pulp, a small percentage of
hypochlorite brings up the desired whiteness under the best conditions for
control. Caustic soda treatment, as a separate step or mixed with bleach
liquor, is applied to dissolve hemi-celluloses. Acid treatment as a last stage
reduces the ash content. It is perhaps superfluous to mention that numerous
modifications of sequence, chemicals, and conditions are practised in both batch
and continuous systems. The significant point is that multi-stage bleaching
has yielded higher strength and purity of bleached sulphite pulps in relation
to whiteness, and has permitted the economic bleaching of strong sulphate
pulps for the benefit of the paper and board industry. The usual loss of fibre
weight in the bleaching of paper-making grades is in the order of 5 per cent
of the unbleached pulp—more or less depending upon the bleactiability and
other conditions—resulting in overall yields of 42 to 45 per cent from the wood
to ordinary bleached sulphite.
Next comes the detailed grading of wood pulps. The dividing lines are
not sharply defined because cooking degree, for instance, must be linked with
strength figures; special characteristics are sometimes the determining factor,
and uses overlap pulp qualities. It should also be remembered that grades are
sometimes interchangeable—for example, semi-bleached sulphate serving in
place of unbleached sulphite for certain papers.
Mechanical pulp grades may be named extra free, free, medium free, and
slow. The drainage rates become progressively slower, due to increasing
hydration and fineness of fibre. In general it can be said that the mechanical