The Kraft process involves cooking wood chips in a solution of sodium hydroxide and sodium sulfide.  The alkaline attack causes fragmentation of the lignin molecules into smaller segments whose sodium salts are soluble in the cooking liquor.  "Kraft" is the German word for strong, and Kraft pulps produce strong paper products.  The unbleached pulp is characterized by a dark brown color.  The Kraft process is associated with malodorous gases, principally organic sulfides.

The Kraft process evolved over 100 years ago as a modification of the soda process (which utilizes only sodium sulfate in the cooking system) when Carl S. Dahl introduced sodium sulfate into the cooking system.  The subsequent conversion of sulfate to sulfide in the cooking liquor produced a dramatic improvement in reaction kinetics and pulp properties when cooking softwoods.  Because sodium sulfate has been the traditional makeup chemical, the kraft process is sometimes referred to as the "sulfate process."  Hardwood pulping is less affected by the presence of sodium sulfide in the alkaline cooking liquor, and some hardwood pulp is still produced by the soda process.

A number of different Kraft pulp grades are produced.  Unbleached grades for packaging applications are cooked to a higher yield and contain more lignin than pulps that are subsequently bleached and made into white papers.  Dissolving Kraft grades, which are used for numerous specialized products including acetate plastics, rayon, and thickeners in food products, are cooked to a lower yield and contain virtually no lignin.

 

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