Abstract: A system (10) for treating spent pulping liquor (14) comprising lignin to provide green liquor (47) is disclosed. The system (10) comprises an evaporator (22), a recovery boiler (34), at least one filter (58, 74), and a green liquor plant (46). The evaporator (22) concentrates a first stream (18) of the spent pulping liquor (14) to provide a concentrated pulping liquor (30). The recovery boiler (34) incinerates the concentrated pulping liquor (30) to provide a smelt (42). The at least one filter (58, 74) filters a second stream (54) of the spent pulping liquor (14) to remove lignin therefrom to provide a permeate (66, 82). The green liquor plant (46) is for dissolving the smelt (42) from the recovery boiler (34) in the permeate (66, 82) from the at least one filter (58, 74) to provide green liquor (47). Also disclosed is a method for treating spent pulping liquor (14) to provide green liquor (47).

Abstract: The present invention describes a method for the treatment of spent pulping liquor for the removal and production of organic substances rich in lignin and recovering or recycling of aqueous cooking chemicals to pulp mill chemicals recovery cycle, said method comprising passing a spent pulping liquor flow from a digester or an evaporator, DF or from any other step in between the digester and the evaporator, through a filtration step in which the flow is separated into one aqueous stream comprising cooking chemicals being recovered or recycled and one stream concentrated with the respect of organic substances; passing the stream concentrated with respect of organic substances either to a lignin depolymerisation step for the production of liquid and pumpable depolymerised lignin or passing the stream concentrated with respect to organic substances to a subsequent filtration step in which an aqueous solution is added and further treatments in order to produce a solid lignin containing product.