sulfite process ( sulfite pulp )
One of the two principal chemical methods of
converting wood into pulp for papermaking, the
other being the SULFATE PROCESS .
Bleached or unbleached sulfite pulp is used in the
manufacture of nearly all classes of paper, and
bleached sulfite pulp is also used in the
manufacture of rayon, cellophane, and other
cellulose esters and ethers. Although it is
possible to produce bleached sulfite pulp from the
hardwoods, the pulp is usually made from softwoods
of low resin content, e.g., spruce, balsam, fir,
and hemlock. Traditionally, sulfite pulping has
involved the digestion of the wood with a calcium
acid sulfite cooking liquor, generally a mixture
of calcium bisulfite and excess sulfurous acid;
however, since processing for waste liquor
recovery is both difficult and impractical with
calcium-base liquors, more soluble bases, such as
sodium, ammonium, and magnesium are now being
substituted for calcium.
The sulfite cooking process may be modified to
produce pulps that can be roughly classified as
soft, medium, or strong, which are classifications
that depend to a great extent on the degree to
which lignin is removed and the cellulose of the
fiber is depolymerized. Sulfite pulping is
superior in the amount of lignin removed, and
produces papermaking fibers that are white in
color and can be bleached to higher whiteness with
less chemicals than required for the sulfate
process. Sulfite fibers also give fewer problems
in maintaining desired formation characteristics
of the finished paper; however, paper made from
sulfite fibers is not as strong as that made from
sulfate pulp. (17 ,
72 , 98 , 143 )
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