Disposal process for sulfide-free black liquors

Abstract

Lignin is flocculated and batched off from a sulfide-free black liquor discharged out of a pulp digestion step by water disposal techniques, and organic matters remaining in the filtrate are removed to recover water, acids and caustic soda. A mineral acid and, if necessary, diluting water is added to the black liquor to control its pH to 1 to 7, and a flocculant is added to the black liquor to filtrate out flocculating lignin. Further, ozone is brought into contact with the filtrate to oxidize and break down organic matters in the filtrate and remaining organic matters are adsorbed onto activate charcoal for removal.

1 . A black liquor disposal process for flocculating and batching off lignin from a sulfide-free black liquor that is discharged out of a pulp digesting step, characterized in that a mineral acid and, if necessary, diluting water is added to the black liquor to control its pH to 1 to 7, and a flocculant is then added to the black liquor for flocculating and batching off lignin. 
   
   
       2 . A black liquor disposal process for flocculating and batching off lignin from a sulfide-free black liquor that is discharged out of a pulp digesting step, characterized in that a mineral acid and, if necessary, diluting water is added to the black liquor to control its pH to 1 to 7, a flocculant is added to the black liquor for flocculating and filtrating lignin, and ozone is brought into contact with a filtrate to oxidize and break down organic matters in the filtrate. 
   
   
       3 . A black liquor disposal process for flocculating and batching off lignin from a sulfide-free black liquor that is discharged out of a pulp digesting step, characterized by carrying out in order:
 (1) a neutralization step of adding a mineral acid and, if necessary, diluting water to the black liquor to control its pH to 6 to 9,   (2) a first flocculation step of adding a flocculant to a liquor obtained in the neutralization step to flocculate lignin,   (3) a first separation step of batching off lignin flocculated in said step (2),   (4) a pH control step of adding a mineral acid and, if necessary, diluting water to a filtrate obtained in said step (3) to control a concentration of solid matters to 1 to 8% by weight and its pH to 1 to 3,   (5) a second flocculation step of adding a flocculant to a liquor obtained in said step (4) to flocculant lignin, and   (6) a second separation step of batching off lignin obtained in said step (5).   
   
   
       4 . The black liquor disposal process as recited in  claim 3 , characterized in that there is an oxidization-by-ozone step applied, in which ozone is further brought into contact with the filtrate batched off in said second separation step to oxidize and break down organic matters in the filtrate. 
   
   
       5 . The black liquor disposal process as recited in  claim 2 , characterized in that the step in which ozone is brought into contact with the filtrate is carried out under ultraviolet irradiation. 
   
   
       6 . The black liquor disposal process as recited in  claim 2 , characterized in that there is an adsorption-by-activated-charcoal step applied, in which activated charcoal is brought into contact with the filtrate leaving the ozone contact step to adsorb and remove organic matters in the filtrate. 
   
   
       7 . The black liquor disposal process as recited in  claim 6 , characterized in that reverse osmosis using a reverse osmosis membrane method is applied to disposal liquor discharged out of the adsorption-by-activated-charcoal step to obtained concentrated salt water and desalted disposal water. 
   
   
       8 . The black liquor disposal process as recited in  claim 7 , characterized in that salt water having a salt content of 80 g/1,000 ml or more is recovered as said concentrated salt water, and there is an electrolysis step applied, in which electrolysis is applied to said salt water to recover the mineral acid and caustic soda. 
   
   
       9 . The black liquor disposal process as recited in  claim 1 , characterized in that said black liquor is obtained by a method in which woody chips are immersed in dilute caustic soda in a hydrophilic treatment step for making them hydrophilic, introducing the chips in a washing step for removal alkalis, oxidizing the chips with dilute nitric acid at normal temperature or with application of heat in an oxidizing step to partially oxidize lignin contained in the chips, introducing the chips in a washing step to wash them, heating the chips under atmospheric pressure with dilute caustic soda in a digesting step to digest the chips, and finally separating the chips into digested pulp and a black liquor containing lignin.