Method for precipitating lignin from black liquor by utilizing waste gases

A method for separation of lignin from original black liquor, comprising:

• precipitating lignin in a first acidification stage of an original black liquor by adding a first acid or mixture of acids and obtaining a first lignin suspension,
• dewatering the first lignin suspension while forming a first filter cake with a high content of lignin,
• suspending the first filter cake in a second acidification stage using a second acid or mixture of acids,
• obtaining a second lignin suspension,
• dewatering the second lignin suspension while forming a second filter cake with a high content of lignin,
• washing the second filter cake,
• dewatering of the washed second filter cake and obtaining a lignin product, re-circulating waste gases emitted from the second acidification stage, and mixing the waste gases with the original black liquor.