Method for handling spent wash solution of a lignin-recovery process

Abstract

A method for handling spent wash solution produced in the washing of lignin that has been precipitated and then separated from spent alkaline pulping liquor. At least part of the spent wash solution is introduced into the chemical recovery cycle at a location in that part of the cycle which starts at, and includes, the smelt dissolving stage and ends at, and includes, the pulping stage. In preferred embodiments, input of other water and/or aqueous solution into the chemical recovery cycle after the recovery boiler is adjusted so that, as a consequence of the introduction of the spent wash solution, the amount of water in the white liquor going to the pulping line is not significantly increased. Among others, two preferred locations for the introduction of the spent wash solution are (1) the smelt dissolver and (2) the location where unwashed lime mud is diluted.

1 . A method for handling spent wash solution produced during washing of lignin that has been precipitated and then separated from spent alkaline pulping liquor at a pulp mill, in which method the separated lignin is washed in at least one washing step being conducted under acidic conditions, wherein at least part of the spent wash solution from the at least one washing step is introduced into a chemical recovery cycle at a location in that part of the cycle which part includes a smelt dissolving stage, a green liquor treatment, white liquor preparation, and a pulping stage. 
     
     
         2 . The method according to  claim 1 , wherein input of other water and/or aqueous solution into the chemical recovery cycle in that part of the cycle which starts at, and includes, the smelt dissolving stage and ends at, and includes, the pulping stage is adjusted so that, as a consequence of the introduction of the spent wash solution, an amount of water in white liquor going from the white liquor preparation to the pulping stage is not significantly increased. 
     
     
         3 . The method according to  claim 1 , wherein sulphuric acid introduced in at least one washing step. 
     
     
         4 . The method according to  claim 1 , wherein spent wash solution is introduced as part of an aqueous medium in the smelt dissolving stage used to dissolve smelt formed in a chemical recovery boiler. 
     
     
         5 . The method according to  claim 1 , wherein the spent wash solution is introduced as part of an aqueous medium to prepare a diluted lime-mud slurry prior to a final washing and dewatering of lime mud. 
     
     
         6 . The method according to  claim 1 , wherein at least part of an aliphatic carboxylic acid fraction dissolved in the spent wash solution is recovered from the solution before the solution is introduced into the chemical recovery cycle. 
     
     
         7 . A method comprising:
 washing lignin under an acidic condition, where the lignin has been precipitated from spent alkaline pulping liquor and a spent wash solution is discharged from the washing of the lignin; and   introducing the spent wash solution into at least one of a smelt dissolving stage, a green liquor treatment stage, a white liquor preparation stage and a pulping stage of a chemical recovery cycle.   
     
     
         8 . The method of  claim 7 , further comprising producing white liquor in the white liquor preparation stage and feeding the white liquor to the pulping stage, and
 introducing an aqueous solution to the chemical recovery cycle in addition to the introduction of the spent wash solution.   
     
     
         9 . The method according to  claim 7 , wherein sulphuric acid is added to the step of washing lignin. 
     
     
         10 . The method according to  claim 7 , wherein the spent wash solution is introduced in the smelt dissolving stage and contributes to dissolving smelt formed in a chemical recovery boiler. 
     
     
         11 . The method according to  claim 7 , further comprising introducing the spent wash to dilute a lime-mud slurry which flows to a lime-mud slurry washing stage. 
     
     
         12 . The method according to  claim 7 , recovering from the spent wash solution an aliphatic carboxylic acid fraction before the introduction of the spent wash solution at least one of the smelt dissolving stage, the green liquor treatment stage, the white liquor preparation stage and the pulping stage.