Method and system for treating spent pulping liquor

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).

1 . A system for treating spent pulping liquor comprising lignin to provide green liquor, the system comprising:
 an evaporator for concentrating a first stream of the spent pulping liquor to provide a concentrated pulping liquor;   a recovery boiler for incinerating the concentrated pulping liquor to provide a smelt;   at least one filter for filtering a second stream of the spent pulping liquor to remove lignin therefrom to provide a permeate; and   a green liquor plant for dissolving the smelt from the recovery boiler in at least a part of the permeate from the at least one filter to provide green liquor.   
     
     
         2 . The system of  claim 1 , wherein the at least one filter comprises:
 a first filter for filtering the second stream of the spent pulping liquor to provide a first retentate and a first permeate; and   a second filter for filtering the first permeate to provide a second retentate and a second permeate;   wherein the second permeate is used to dissolve the smelt in the green liquor plant.   
     
     
         3 . The system of  claim 2 , wherein the first filter is a membrane filter with a molecular weight cut-off between 0.5 and 20 kDa and the second filter is a membrane filter with a molecular weight cut-off lower than first filter. 
     
     
         4 . The system of  claim 2 , wherein the first filter is a membrane filter with a molecular weight cut-off of about 1 kDa. 
     
     
         5 . The system of  claim 1 , wherein the second stream is between about 10 to 50% by volume of the spent pulping liquor. 
     
     
         6 . The system of  claim 1 , wherein the second stream is about 25% by volume and the first stream is about 75% by volume of the spent pulping liquor. 
     
     
         7 . The system of  claim 1 , further comprising a de-polymerization reactor for treating the first retentate to provide a de-polymerized lignin stream. 
     
     
         8 . A method for treating spent pulping liquor comprising lignin to provide green liquor, the method comprising:
 (a) dividing the spent pulping liquor into a first stream and a second stream;   (b) evaporating the first stream to provide a concentrated pulping liquor;   (c) incinerating the concentrated pulping liquor to provide a smelt;   (d) filtering the second stream to remove lignin, thereby obtaining a permeate with reduced content of lignin;   (e) dissolving the smelt in at least part of the permeate to provide green liquor.   
     
     
         9 . The method of  claim 8 , wherein step (d) filtering the second stream to obtain a permeate comprises:
 (d1) filtering the second stream of the spent pulping liquor to remove a first portion of lignin to provide a first retentate and a first permeate; and   (d2) filtering the first permeate to remove a second portion of lignin to provide a second retentate and a second permeate;   wherein at least part of the second permeate is used to dissolve the smelt in the green liquor plant in step (e).   
     
     
         10 . The method of  claim 8 , wherein the second stream is between about 10 to 50% by volume of the spent pulping liquor. 
     
     
         11 . The method of  claim 8 , wherein the pH of the permeate used to dilute the smelt is at least 11. 
     
     
         12 . The method of  claim 8 , wherein part of the permeate is sent to the evaporator. 
     
     
         13 . The system of  claim 3 , wherein the molecular weight cut-off of the second filter is between 0.1 and 0.8 kDa. 
     
     
         14 . The method of  claim 10 , wherein the second stream is about 25% by volume of the spent pulping liquor. 
     
     
         15 . The method of  claim 11 , wherein the pH of the permeate used to dilute the smelt is at least 12.