Semichemical

Semichemical is a term used in the papermaking industry to describe a class of pulp produced by partial chemical treatment of wood followed by mechanical processing. The approach lies between mechanical pulps, which rely largely on physical fiber separation, and chemical pulps, which are fully cooked to remove lignin. Semichemical pulps are designed to combine high fiber yield with improved strength over purely mechanical pulps, while avoiding the full chemical costs and environmental load of conventional chemical pulps.

Production typically begins with cooking wood chips in a mild chemical liquor to remove part of the lignin and some extractives. After this partial cook, the material is subjected to mechanical refining to separate fibers and develop bonding areas. The resulting pulp retains more lignin than fully chemical pulps, but with a cleaner fiber surface than untreated mechanical pulps. Additional bleaching is possible for brighter grades.

Semichemical pulps are commonly used for packaging applications, especially linerboard and corrugated medium, as well as certain specialty papers where higher strength and stiffness are valued. They are chosen for their favorable balance of yield, strength, and process economics.

Environmental and economic considerations include lower chemical consumption and effluent load compared with full chemical pulping, offset by energy use in refining and mechanical processing. Variants of semichemical pulping may employ different cooking chemistries, including alkaline sulfite or kraft-based approaches.

Related topics include mechanical pulp, chemical pulp, sulfite process, kraft process, and pulp refining.