WOOD PULP 57
fitted with thermometer tubes, safety valves, blow-off cocks, etc., and also cocks
for drawing samples of the liquor and pulp for testing purposes.
The bisulphite liquor for digesting the wood is made in the following
way:
Sulphur or iron pyrites is burned in an oven with a carefully regulated
supply of air. In this way sulphur dioxide gas is formed, which, after being
cooled, is passed into the bottom of a tower, in which is stacked ordinary
limestone. Water is made to trickle down and over the limestone, and the
gas passing upwards mixes freely with it. The sulphur dioxide, or SO2, is
absorbed by the water, H»O, and forms sulphurous acid, HaSO3:
H.O+SO.=H,SO,.
This solution acts on the limestone (carbonate of lime) to form sulphite of lime,
water, and carbon dioxide:
H2SOa+CaCO3=CaSO3+H2O+CO2.
Ultimately bisulphite of lime is formed by the further solution of limestone.
During the reaction free sulphurous acid is also formed, and it is this acid which
is required for the reduction of the wood.
The sulphite process is largely confined to softwoods of low resin content.
Notable advances are its advancement to pine of medium resin content, and
its increasing application to hardwoods. From the chemical point of view
the sulphite process represents a comparatively severe hydrolysis of the fibre
in removing lignin and part of the lower carbohydrates from the wood. The
pulp fibres are very flexible, but some strength is sacrificed. The great ad-
vantages are bright colour of the unbleached pulp for direct use and ease of
bleaching, coinciding with the unconscious desire of the buying public for
white papers. Particularly in Europe long experience in operation and wide-
spread adoption of digester circulation has resulted in high quality of sulphite
pulps. The yield of unbleached sulphite for paper-making usually varies from
45 to 50 per cent by weight of the chips, depending on wood species, bleach-
ability, and other factors. This is a considerable range, amounting to 10 per
cent difference in tonnage from the same weight of wood. There is no certainty
that world tonnage of unbleached sulphite will materially increase, but
bleached sulphite presumably will extend, for chemical uses in particular.
Here may be mentioned some of the modified sulphite processes. The
Mitscherlich method of indirect heating and long cooking has become too
expensive for general competition. The minimum change in the standard
sulphite cooking liquor is the use of magnesium base in pkce of calcium base.
Sodium bisulphite in America and Scandinavia is applied to resinous woods