How does semichemical pulping combine chemical and mechanical methods?

Semichemical pulping is a hybrid process that integrates chemical pretreatment with mechanical reduction to produce paper pulp.1

The process combines these methods through the following steps:

• Chemical Pretreatment: Wood chips are steeped and impregnated with chemical solutions, such as sodium sulfite, caustic soda, or kraft cooking liquor. Unlike full chemical pulping, these chemicals are used in smaller quantities and under less severe conditions, which softens the lignin within the wood without dissolving it entirely.23
• Mechanical Reduction: Once the chips are softened, they are fed into disk refiners. The attrition action of these refiners mechanically reduces the softened chips into individual fibers.21

This combination results in a higher fiber yield—typically 66 to 90 percent—compared to purely chemical methods. The resulting pulp possesses strength and chemical properties that fall between those of mechanical groundwood and full chemical pulps, making it ideal for products like corrugating medium used in heavy-duty shipping containers.21