Kraft Sulfate Lignin

Springer Series in Wood Science Methods in Lignin Chemistry (Edited by S.Y. Lin and C.W. Dence) 

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By means of Fremy salt oxidation, Adler and Lundquist (5) estimated that about half of the phenolic units contain free S-positions in M.W.L. Mikawa (J/), by Mannich reaction, obtained evidence that about 25-40% of the phenolic units in his sulfate lignin preparation contained no substituent in 5-position. In this investigation we also conclude that at least one-third of the phenolic units in our kraft lignin preparation has free 5-positions. 

Commercially, most cellulose is extracted from wood by one of two methods, the kraft (sulfate) process or the steam explosion process. The product of these reactions is wood pulp, which consists primarily of cellulose. In the kraft process, wood chips are treated with a solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S) at temperatures of about 175°C (35o°F) for two to six hours. This process usually results in a yield of about 40 to 45 percent wood pulp. The pulp is then treated with a bleaching agent, such as calcium or sodium hypochlorite (Ca(0Cl)2 or NaCIO) or chlorine dioxide (C102) to remove the color of lignin and other impurities. 

Alkaline delignification in the form of the Kraft or Sulfate process is now the most widely used method of lignin removal. It uses a mixture of sodium hydroxide and sodium sulfide—the latter being produced in the recovery process by the reduction of sodium sulfate... 

Recovery of inorganic chemicals is crucial to the cost effectiveness of the Kraft process. The black liquor which is obtained from delignification is rich in solubilised lignin and carbohydrate degradation products and, after concentration, is combusted in a recovery furnace. The Carbon dioxide which is produced during combustion converts unused sodium hydroxide into sodium carbonate. In addition, the sodium sulfate is converted, under the reducing atmosphere of the furnace, to sodium sulfide. 

The process of pulping, degrading the lignin to a more soluble form so the cellulose fibers can be separated from it, involves some interesting chemistry. The kraft or alkaline sulfate process dominates this part of the industry. Approximately 78% of all pulp is made by the kraft process, 3%... 

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