Method for separating at least one lignin fraction, lignin derivatives, low-molecular carboxylic acids, inorganic salt, or a mixture thereof from filtrates originating from a kraft process

A method for separating water and at least one lignin fraction, lignin derivatives, low-molecular carboxylic acids, sodium sulfate or a mixture thereof from filtrates originating from black liquor from a kraft process, wherein

• before a first separation step, the black liquor is concentrated to a solids content of between 25 and 55 wt. % solids content,
• in the first separation step, polymeric lignin with a number average of the molecular mass distribution >6 kDa is precipitated and filtered off by lowering a pH of black liquor to a value in the a range of pH 8 to pH 10, whereby a first filtrate is obtained in addition to lignin with a number average of the molecular mass distribution >6 kDa,
• the first basic filtrate obtained is acidified with sulfuric acid,
• a precipitate formed consisting of lignin with a number average of the molecular mass distribution of <6 kDa is separated by filtration and a second acidic filtrate is obtained;
• the second filtrate is cooled to a temperature of less than 25° C., whereby a fraction consisting substantially of inorganic salts, is crystallized out, after addition of seed crystals;
• filtering off the precipitate formed, which consists substantially of inorganic salts, whereby a third filtrate containing low-molecular carboxylic acid-soluble lignin and residues of the inorganic salts is obtained;
• returning dried sodium sulfate to the third filtrate and carrying out a second crystallization of Na2SO41.0H2O;
• and/or
• returning the third filtrate into a pulp production process or
• carrying out a reactive extraction of the third filtrate;
• evaporating or distilling the third filtrate to obtain a product condensate containing a first fraction containing a plurality of different organic low-molecular acids and a product residue containing at least organic acids and/or polyphenols.