Method for separating water and at least one lignin fraction from black liquor filtrates of the Kraft process

Abstract

In a process for separating water and at least one lignin fraction, lignin derivatives, low molecular weight carboxylic acids, sodium sulfate or a mixture thereof from filtrates originating from black liquor of a Kraft process, the black liquor is reduced to a solids content between 25 and 55 wt.-% solids content concentrated, in the first separation step polymeric lignin with a number-average molar mass distribution >6 kDa is precipitated by lowering the pH of black liquor and filtered off, thereby obtaining a first filtrate in addition to lignin with a number-average molar mass distribution >6 kDa, the obtained filtrate is acidified with sulfuric acid, a precipitate consisting of lignin with a number-average molar mass distribution of <6 kDa is separated by filtration and a second acidic filtrate is obtained, the second filtrate is cooled to a temperature below 10 °C, whereby a fraction consisting essentially of inorganic salts is crystallized; filtration of the formed precipitate yields a third filtrate containing low-molecular-weight carboxylic acids soluble lignin as well as residues of the inorganic salts.