The Kraft process (also known as Kraft pulping or sulfate process) is used in production of paper pulp and involves the use of caustic sodium hydroxide and sodium sulfide to extract the lignin from the wood fiber in large pressure vessels called digesters. The spent pulping liquor, called black liquor, is concentrated by evaporation and burned to generate high pressure steam for the mill processes. The inorganic portion of the black liquor is then used to regenerate the sodium hydroxide and sodium sulphide needed for pulping. In the case of softwood (conifer) pulping, a soaplike substance is collected from the liquor during evaporation. The soap is acidified to produce tall oil, a source of resin acids, fatty acids and other chemicals. Another byproducts are the lignosulfonates, important deflocculants in drilling muds for oil industry.

The process name is derived from German kraft, meaning strong; both capitalized and lowercase names (Kraft process and kraft process) appear in the literature. It was developed by Carl Dahl in 1884 and now is used for about 80% of production volume of paper.

Kraft process differs from the sulfite process by using alkaline solution, which is less corrosive to the equipment. It is also more efficient than the sulfite process. It produces stronger fiber, however the fiber is also rougher and contains more impurities that make it difficult to bleach.

Various gaseous byproducts containing hydrogen sulfide and other volatile sulfur compounds are the cause of the obnoxious smells characteristic for pulp and paper mills utilizing the kraft process.

External link

• US EPA article on Kraft pulping

Industrial processes | Pulp and paper industry