Method for producing high purity lignin

Abstract

The method is for separation of lignin from original black liquor (BL IN ) that has a first precipitation stage (PR) for precipitation of lignin by a first acidification using acidifier AC 1 , preferably using CO 2 , at alkaline conditions, then separating a lignin cake with subsequent suspension of the lignin cake in a strong acid in order to leach our metals from the lignin followed by dewatering and obtaining a clean lignin product LP. The process further is improved by intensified hydrolysis of lignin cake such that most of the carbohydrates are broken down to dissolvable monomers that could be separated from the lignin in the filtrate from a filtering stage subsequent to the hydrolysis. The improved hydrolysis could reduce as much as 90% of the carbohydrate content using a moderately increased temperature and increased charge of acidifier while avoiding any larger lignin yield losses.

I claim: 
     
         1 . A method for separation of lignin from original black liquor (BL IN ) having a first alkaline pH value, comprising the following stages in sequence:
 in a first precipitation stage (PR), adding an acidifier charge to an original black liquor in order to decrease a pH value of the original black liquor to a second pH level initiating precipitation of lignin so that the second pH level is above a pH value of 7 and below 11.5,   in a subsequent first separation stage (FP 1 ), separating a precipitated lignin as a lignin cake (LIG 1 ) with a content of carbohydrates from a remaining liquid phase of the original black liquor still kept in a pH range from neutral to alkaline,   mixing a lignin from the lignin cake with a content of carbohydrates in a subsequent stage with a second acidifier added to the lignin cake forming an acidic slurry establishing a pH value in a range of 1-3,   establishing a reaction temperature in a range of 100-140° C. in the acidic slurry,   maintaining the acidic slurry at the reaction temperature for a reaction time period during which at least 60% of the content of carbohydrates is hydrolyzed, the reaction time period resulting in a P-factor of less than 20, and   in a subsequent second separation stage, separating a treated lignin content from the acidic slurry and carbohydrates dissolved in the acidic slurry forming a low carbohydrate lignin cake.   
     
     
         2 . A method according to  claim 1  wherein the reaction temperature in the acidic slurry is in a range of 100-120° C. and the reaction time period during which at least 60% of the content of carbohydrates is hydrolyzed is in a range of 10-60 minutes in inverse proportion to the reaction temperature, keeping an amount of lignin dissolved from the lignin cake below 15%. 
     
     
         3 . A method according to  claim 1  wherein after the acidic slurry has been kept at the reaction temperature for a reaction time period during which at least 60% of the content of carbohydrates is hydrolyzed, the acid slurry is subjected to cooling before subsequent separation of the treated lignin content. 
     
     
         4 . A method according to  claim 2  wherein after the acidic slurry is cooled in an indirect heat exchanger (HE) against an acidifier to be used as a second acidifier, reducing the temperature of the acidic slurry by at least 40° C. 
     
     
         5 . A method according to  claim 3  wherein the acidic slurry has been kept at the reaction temperature for a reaction time period during which at least 60% of the content of carbohydrates is hydrolyzed, the reaction temperature and time period corresponding to a P-factor not exceeding a P-factor established at a reaction temperature of 120° C. and a time period of 60 minutes. 
     
     
         6 . A method according to  claim 5  wherein the acidic slurry has been kept at the reaction temperature for a reaction time period during which at least 90% of the content of carbohydrates is hydrolyzed, the reaction temperature and time period corresponding to a P-factor equivalent to or exceeding a P-factor established at a reaction temperature of 100° C. and a time period of 60 minutes. 
     
     
         7 . A method according to  claim 1  wherein the lignin cake is subjected to an additional acidification to a pH of at least 2-4 or lower followed by a third separation of a third lignin cake as an additional treatment step for leaching and washing metals from the third lignin cake, the additional treatment made either before or after obtaining the low carbohydrate lignin cake. 
     
     
         8 . A method according to  claim 1  wherein the lignin cake is washed with washing water at a pH of at least 2-4 or lower after at least one of the separation stages.