Kraft process

The kraft process is a chemical pulping method that separates cellulose fibers from wood by cooking wood chips in an alkaline solution of sodium hydroxide and sodium sulfide.

White liquor: An aqueous pulping solution whose principal active chemicals are sodium hydroxide and sodium sulfide. Its hydroxide and sulfide ions break down lignin during kraft cooking.
Kraft paper: Strong paper made primarily from kraft pulp, commonly used for bags, wrapping, and packaging. Its strength makes it a familiar product of unbleached kraft pulp.
Carl F. Dahl: A German chemist who developed the kraft pulping process in the 1870s. Dahl introduced sodium sulfate to chemical recovery, leading to the process's name and chemistry.
Chemical recovery boiler: A boiler that burns concentrated black liquor to generate steam and recover inorganic pulping chemicals as smelt. It recovers energy and chemicals, but is also a major mill emissions source.
Kraft cooking: The high-temperature treatment of wood chips with white liquor to dissolve much of their lignin. This is the central reaction stage that releases fibers from the wood.
Paperboard: A thick, stiff paper-based material used to make cartons, boxes, and other packaging. Kraft fibers provide strength in many grades of packaging board.
Sulfate process: An early name for kraft pulping, derived from sodium sulfate used to replenish cooking chemicals. The name reflects the recovery route associated with Dahl's innovation.
Smelt dissolving tank: A vessel where molten recovery-boiler smelt is dissolved in water to form green liquor. Its operation connects chemical recovery to regeneration of the cooking liquor.
Lignin: A complex aromatic polymer that strengthens plant cell walls and binds cellulose fibers. Removing lignin lets the cellulose-rich fibers separate during cooking.
Dissolving pulp: Highly purified cellulose pulp used to manufacture regenerated cellulose and cellulose derivatives. Modified kraft processes can produce this pulp for textiles and chemical products.

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