Method of converting wood chips to paper pulp for papermaking utilizing a combination of chemical and mechanical means. The purpose of pulping is to separate individual fibers of cellulose from other non-fibrous components of wood, in particular, lignin, an organic material that binds cellulose fibers together, and is a detriment to papermaking. Wholly mechanical pulping generates high pulp yields, and costs comparatively little, but removes little lignin. Wholly chemical pulping removes most lignin, but is expensive and has low pulp yields.
In semichemical pulping, wood chips are first subjected to mild cooking in, most commonly, sodium sulfite combined with a small quantity of alkaline salts, such as sodium carbonate, sodium bicarbonate, or sodium hydroxide. The cooked chips are then sandwiched in a disk refiner—or two rotating serrated disks—that separate the individual fibers of cellulose. The pulp is then washed to remove the chemicals.
Some of the advantages of chemical pulping, including higher lignin removal, and of mechanical pulping, such as high pulp yields, are realized. Pulp yields in semichemical processes are generally from 60:80% of the original wood, and much of the residual lignin still remains. Semichemical pulping results in stiff fibers, and the process is used to make corrugated paperboard, cardboard roll cores, and containers.