Reduction of organically bound chlorine formed in chlorine dioxide bleaching

Abstract

A reduction in AOX levels is obtained when the process conditions in the chlorine dioxide stage are elevated to above 91° C. and extended to more than 90 minutes. A major reduction of AOX up to 50% has been shown without a corresponding increase in OCl. The chlorinated substances is degraded by the process conditions to harmless chloride ions, instead of being liberated into the effluent as AOX or bound to pulp as OCl. No oxygen gas or nitrogen gas are added during the first or any subsequent chlorine dioxide bleaching stage.

1 . A process for reducing the amount of organically bound chlorine formed in chlorine dioxide bleaching of kraft pulp, comprising: 
 providing several bleaching stages wherein at least one of the stages is a bleaching stage using chlorine dioxide as bleaching chemical to form a bleaching sequence;    in a first chlorine dioxide bleaching stage, charging a first amount of chlorine dioxide to the pulp having a first kappa number so that a charge factor of the first amount of chlorine dioxide/first kappa number is above 0.5 and operating at a temperature above 91 C and a retention time longer than 90 minutes;    reducing a resulting AOX content in an effluent from the bleaching sequence to a value below 0.23 kg/ADt; and    adding no oxygen gas or nitrogen gas to the first chlorine dioxide bleaching stage.    
     
     
         2 . The process according to  claim 1  wherein the retention time is less than 300 minutes.  
     
     
         3 . The process according to  claim 1  wherein the first chlorine dioxide bleaching stage used during the bleaching sequence is operated at a temperature between 95° C. and 120° C. at a retention time of between 90 minutes and 300 minutes and the first chlorine dioxide bleaching stage is pressurized to a pressure exceeding the vapor saturation pressure for the temperature in the stage by at least 20%.  
     
     
         4 . The process according to  claim 1  wherein the charge factor is in the range 1.5-3.0.  
     
     
         5 . The process according to  claim 1  wherein the first chlorine dioxide bleaching stage has a pulp concentration between 7-25%.  
     
     
         6 . The process according to  claim 1  wherein the process comprises the steps of delignifying the pulp to a kappa number below 20 prior to bleaching the pulp in the first chlorine dioxide bleaching stage.  
     
     
         7 . The process according to  claim 6  wherein the method further comprises adding a sulphamic acid to at least one of the chlorine dioxide bleaching stages in the bleaching sequence to capture intermediately formed chlorine or hypochlorite to form a chlorosulphamic acid.  
     
     
         8 . The process according to  claim 7  wherein the method further comprises adding the sulphamic acid in a mmol amount that is greater than a mmol amount of active chlorine dioxide added during the first chlorine dioxide bleaching stage.  
     
     
         9 . The process according to  claim 7  wherein the method further comprises adding at least 1.0 mmol sulphamic acid/BDt of pulp.  
     
     
         10 . The process according to  claim 7  wherein at least 80% of all sulphamic acid added is added to a last chlorine dioxide bleaching stage.  
     
     
         11 . The process according to  claim 10  wherein the last chlorine stage is a D2 stage preceded by a D0 chlorine stage and a D1 chlorine stage and the method further having extraction stages E between the chlorine dioxide stages to form a D0-E-D1-E-D2 bleaching sequence.  
     
     
         12 . The process according to  claim 11  wherein a charge of chlorine dioxide, used in a chlorine stage including sulphamic acid, is at least 10% greater than a charge of chlorine dioxide used in a chlorine stage that is free from sulphamic acid.