Method for recovering a low sodium content lignin fuel from black liquor

A method for recovering low sodium content lignin from spent kraft cooking liquor without the addition of a strong mineral acid for acidulation of said liquor comprising the steps of:

• a) acidulating a spent kraft cooking liquor comprising solubilized lignin with carbon dioxide gas to a pH below about 10.5 thereby forming sodium carbonate and sodium bicarbonate salts that solubilizes in said spent liquor wherein said acidulation has caused the precipitation of at least a portion of lignin in spent kraft cooking liquor forming lignin agglomerates and lignin particles in said spent liquor;
• b) treating spent liquor comprising precipitated lignin from step a) by one or more of filtering, washing filtration cake with a washing liquid, and dewatering, in at least two treatment stages, wherein at least one washing liquid comprises calcium or magnesium ions, said calcium or magnesium ions replacing at least a portion of sodium bound to lignin;
• c) forming at least two separate streams of spent washing liquids from washing and dewatering performed in step b) wherein at least one spent washing liquid contains calcium or magnesium compounds originating from washing liquids used in step b);
• and
• d) recovering by filtration a low water and low sodium content lignin product comprising calcium or magnesium bound to said lignin.