Kraft process

A kraft paper mill in The Pas, Manitoba

Woodchips for paper production

The kraft pulping process, or simply kraft or sulfate process (spelling, sulphate in the U.K. and older literature) is a preeminent industrial process for conversion of wood into wood pulp, a product that consists of almost pure cellulose fibres, the main component of paper. The process name is derived from the Swedish and German word Kraft, meaning "strength" in this context, due to the characteristic strength of the kraft paper produced using the process.

The kraft process involves treatment of wood chips with a heated "white liquor"—a mixture that includes sodium hydroxide and sodium sulfide dissolved in water—that breaks the chemical bonds that link the branched wood heteropolymers lignin and hemicellulose, and homopolymeric cellulose. The process entails several steps, both chemical and mechanical, and it is the predominant process globally and within the U.S. for producing the wood pulp used in the manufacture of paper. In some locales, manufacturing using the process has been controversial because kraft plants release odorous volatiles and produce and store substantial liquid chemical intermediates and waste.