In chemical pulping, wood chips are cooked with appropriate chemicals is an aqueous solution at elevated temperature and pressure. The objective is to degrade and dissolve away the lignin and leave behind most of the cellulose and hemicelluloses in the form of intact fibers. In practice, chemical pulping methods are successful in removing most of the lignin. They also degrade and dissolve a certain amount of the hemicelluloses and cellulose so that the yield of pulp is low relative to mechanical pulping methods, usually between 40-50% of the original wood substance.
The two principle chemical pulping methods are the (alkaline) Kraft process and the (acidic) sulfite process. The Kraft process has come to occupy the dominant position because of advantages in chemical recovery and pulp strength. The sulfite process, which was more common up to the late 1940’s, appears to be in an irreversible decline. No new sulfite mills have been built in North America since 1960’s. Nonetheless, sulfite pulping still exists, and process modifications have been proposed which would appear to make the process more competitive.