pulping

Pulping is the process of converting lignocellulosic biomass, most commonly wood, into a pulp—a slurry of cellulose fibers suitable for papermaking. The process separates cellulose fibers from lignin and other constituents, producing material that can be further refined, bleached, and formed into paper or other fiber products.

Pulping methods are broadly categorized as mechanical and chemical. Mechanical pulping physically frees fibers with grinding or refining, preserving much of the original lignin but delivering high yields and requiring significant energy; the resulting pulp is typically darker and weaker. Chemical pulping uses chemicals to dissolve lignin and separate fibers, yielding stronger, brighter pulp with lower yield but higher process flexibility.

Common chemical pulping processes include kraft (sulfate) and sulfite methods. Kraft is dominant in modern pulp production, using sodium hydroxide and sodium sulfide to dissolve lignin, with lignin-rich black liquor recovered for energy. Semi-chemical pulping combines chemical pretreatment with mechanical refining. Thermomechanical pulping uses steam to soften fibers before mechanical defibration.

The pulp may require washing and bleaching to remove residual chemicals and lignin, with bleaching increasingly moving toward chlorine-free sequences to reduce environmental impact (ECF and TCF processes). Pulp is then dried or formed into paper grades, packaging board, or dissolving pulp for chemical derivatives.

Feedstock can include non-wood fibers such as bamboo, bagasse, or straw. Pulping is central to the paper industry and to the production of cellulose-based products, with environmental considerations including energy use, chemical recovery, effluents, and forest management.