BigPatents India

A process for the selective separation of high molar mass fraction of lignin from the black liquor discharged from small agro-based pulp and paper mills in order to reduce the pollution load of the discharged effluent laden with lignin rich black liquor in terms of COD and colour is disclosed. Removal of this high molar mass fraction of lignin further improves the treatability of the resultant liquor after removal of lignin, thereby helping the mills to achieve desired standards nearer to the specified norms for COD and colour. The process involves selective precipitation and flocculation of the high molar mass fraction of lignin by addition of an identified coagulant and a flocculant to an optimized pH level which is below neutral and temperature of the system is maintained between 50-70oC which results in precipitation of 60-65% of lignin in the form of high molar mass fraction. The precipitated lignin thus obtained has improved filterability, which is separated from the liquor.