Reduction of organically bound chlorine formed in chlorine dioxide bleaching

Abstract

The invention relates to an improvement in bleaching of kraft pulp. In ECF bleaching chlorine dioxide is most often a prefered bleaching chemical in certain process positions. The disadvantage is that residual amounts of organically bound chlorine in form of AOX, i.e organically bound chlorine in effluents, and/or OCl, i.e. chlorine organically bound in the produced pulp, is obtained. A normal approach has been to reduce charges of chlorine dioxide. According to the invention could substantial reductions in AOX levels be obtained if the process conditions in the chlorine dioxide stage elevated to above 91° C. and extended to more than 90 minutes. A major reduction of AOX up to 50% have 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.

1 . A process for reducing the amount of organically bound chlorine formed in chlorine dioxide bleaching of kraft pulp using several bleaching stages and wherein at least one of the stages is a bleaching stage using chlorine dioxide as bleaching chemical, said bleaching stages forming a bleaching sequence, wherein a first chlorine dioxide bleaching stage used during the bleaching sequence is having a charge factor above 0.5 characterised in that the first chlorine dioxide bleaching stage used during the bleaching sequence is operated at a temperature above 91° C. and at a retention time more than 90 minutes, whereby the resulting AOX content in the effluent from the bleaching line is reduced more than 25%.  
     
     
         2 . A process according to  claim 1 , characterised in that the that the first chlorine dioxide bleaching stage used during the bleaching sequence is operated at a temperature above 95° C. up to 120° C. at the most and at a retention time more than 90 minutes up to 300 minutes at the most, and preferably about 200 minutes, and that the first chlorine dioxide bleaching stage is pressurised to a pressure exceeding the vapour saturation pressure for the temperature in the stage by at least 20%.  
     
     
         3 . A process according to  claim 1  or  2 , characterised in that the charge factor in the first chlorine dioxide bleaching stage is above 1.5 and preferably in the range 1.5-3.0.  
     
     
         4 . A process according to any of claims  1 - 3 , characterised in that the pulp concentration during the the first chlorine dioxide bleaching stage is in the medium consistency range, 25 i.e. between 7-25%.  
     
     
         5 . A process according to any of claims  1 - 4 , characterised in that the pulp being bleached in the first chlorine dioxide bleaching stage is delignified to a kappa number below 20, and preferably below kappa 15 prior to bleaching in the first chlorine dioxide bleaching stage.  
     
     
         6 . A process according to  claim 5 , characterised in that sulphamic acid is added to at least one chlorine dioxide bleaching stage in the bleaching sequence, which sulphamic acid captures intermediately formed chlorine, or hypochlorite, during the chlorine dioxide bleaching stage forming chlorosulphamic acid according to the reaction process.  
     
     
         7 . A process according to  claim 6 , characterised in that sulphamic acid is added to at least one chlorine dioxide bleaching stage in the bleaching sequence in an amount exceeding that of the charge of active chlorine based upon a mmol relation.  
     
     
         8 . A process according to  claim 6 , characterised in that sulphamic acid is added to at least one chlorine dioxide bleaching stage in the bleaching sequence in an amount exceeding 1.0 mmol sulphamic acid/BDt of pulp.  
     
     
         9 . A process according to  claim 6 , characterised in that the major part, i.e. more than 80% of total charge and preferably 100%, of the total charge of of sulphamic acid added to the bleaching sequence is added to the last chlorine dioxide bleaching stage.  
     
     
         10 . A process according to  claim 9 , characterised in that the last chlorine dioxide bleaching stage is at least a D2 stage, i.e. a chlorine dioxide stage preceeded by at least a D0 stage, i.e. the first chlorine dioxide stage, and a D1 stage, having extraction stages between chlorine dioxide stages, i.e. according to a D0-E-D1-E-D2 bleaching sequence.  
     
     
         11 . A process according to  claim 8 ,  9  or  10  characterised in that the charge of chlorine dioxide used in the stage where sulphamic acid is added is increased by at least 10%, preferably increased 20%, as compared to a charge of chlorine dioxide used in this stage without addition of sulphamic acid and resulting in a final pulp brigthness in the same order of ISO brightness, i.e. the same order of ISO brightness corresponing to ±1% in final ISO brightness.