Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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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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