A chemical pulping method of converting wood chips into paper pulp. The purpose of pulping is to reduce wood (or other fibrous raw material) to individual cellulose fibers. A non-fibrous constituent of wood, lignin, binds cellulose fibers together, and is primarily responsible for reducing paper quality and permanence. The sulfite process, the older and less used of the two primary chemical pulping processes, cooks wood chips in sulfurous acid combined with limestone to produce calcium bisulfite. The combination of sulfurous acid and calcium bisulfite dissolves the lignin in the wood and liberates the cellulose fibers. Sulfite pulp is soft and flexible, is moderately strong, and is used to supplement mechanical pulps (most typically in newsprint). The sulfite was the most prevalent chemical pulping process until the mid-twentieth century. The sulfite process is not well-suited to pulping highly resinous wood, such as pine trees. Its original calcium-based chemicals are unrecoverable, and pollution laws have resulted in newer substances being utilized, such as sodium, magnesium, and ammonia bases (the active chemical in the sulfite process is the bisulfite ion (HSO3-). These bases are easier to recover. The sulfite process is an acidic pulping process, and exists in contrast to the alkaline soda process. The sulfite process has been replaced in very large part by the sulfate or kraft process. Pulps produced using the sulfite process are called sulfite pulp. (See Chemical Pulping and Kraft.)
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