Isolation Procedure for Kraft Lignin

The following steps can be used to isolate fractions of kraft lignin from black liquor. 

Heat 40% black liquor to 90-95 °C in a hot water bath under rapid stirring. 

Acidify the hot liquor carefully with 20% (by weight) sulfuric acid to pH 11 and cool to 40 °C in an ice bath. 

Filter the acidified liquor through a Whatman No. 4 filter paper. Recover the filter cake. 

Reslurry the filter cake in distilled water and under good stirring acidify the slurry to pH 2 with 20% sulfuric acid. Centrifuge off the precipitated lignin. After several washings with water at pH 2, air dry the recovered lignin. 

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