Kraft process

The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp consisting of almost pure cellulose fibers. It entails treatment of wood chips with a mixture of sodium hydroxide and sodium sulfide, known as white liquor, that breaks the bonds that link lignin to the cellulose.

History

The kraft process (so called because of the superior strength of the resulting paper, from the German word Kraft for 'strength') was invented by Carl F. Dahl in 1879 in Danzig, Prussia, Germany. U.S. Patent 296,935 was issued in 1884, and a pulp mill using this technology started (in Sweden) in 1890. The invention of the recovery boiler by G.H. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process. It enabled the recovery and reuse of the inorganic pulping chemicals such that a kraft mill is a nearly closed-cycle process with respect to inorganic chemicals, apart from those used in the bleaching process. For this reason, in the 1940s, the kraft process superseded the sulfite process as the dominant method for producing wood pulp.