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chemical process for the manufacture of paper pulp that employs an acid bisulfite solution to soften the wood material by removing the lignin from the cellulose. Sulfite cooking liquor used in the process consists of free sulfur dioxide obtained by the burning of sulfur or by the roasting of iron pyrites, dissolved in water at a concentration of four to eight percent, with from two to three percent in the form of bisulfite. The sulfite digestion of the wood material is normally carried out as a batch process in a pressure vessel that consists of a steel shell possessing an acid-resistant lining. See also kraft process.