Extended retention and medium consistency pulp treatment

Abstract

Treatment of digested or O2 delignified cellulosic pulp in a chlorine dioxide (D o ) delignification stage, preceding bleaching of the pulp, at a medium pulp consistency for an extended period of time. Unexpected benefits include improvement in the delignification efficiency and selectivity, a lowering of the chlorine dioxide consumption in the overall post-digestion or post-O2 delignification and bleaching process, a reduction in the filtrate volume, a reduction of COD/AOX from an ECF bleaching plant, improved pulp properties, and enhanced removal of Hex-A. The process may be implemented in existing three, four or five-stage bleaching plants.

1 . In a process for the treatment of digested cellulosic pulp preparatory to bleaching of the pulp, the improvement comprising subjecting the pulp at a medium consistency to chlorine dioxide for a time period of at least 60 minutes to delignify the pulp prior to bleaching thereof.  
     
     
         2 . The improvement of  claim 1  wherein the pulp at a medium consistency is subjected to chlorine dioxide for a time period between about 60 minutes and about 180 minutes.  
     
     
         3 . The improvement of  claim 1  wherein the pulp is at a consistency of between about 10% and about 15%, based on the weight of oven dried pulp.  
     
     
         4 . The improvement of  claim 1  wherein the pulp is prewashed following digestion thereof and prior to subjection of the pulp to chlorine dioxide.  
     
     
         5 . The improvement of  claim 1  and including the step of subjecting said digested pulp to O2 delignification prior to subjecting said pulp to chlorine dioxide.  
     
     
         6 . The improvement of  claim 1  wherein said chlorine dioxide is in either a liquid or gaseous state.  
     
     
         7 . The improvement of  claim 1  wherein said pulp comprises either hardwood or softwood pulp.  
     
     
         8 . The improvement of  claim 1  and including the further step of subjecting said first treated pulp to one or more bleaching operations for enhancing at least the brightness of the pulp and removal of dirt from the pulp.  
     
     
         9 . The improvement of  claim 8  wherein said bleaching operations include sequentially subjecting the first treated pulp to an extraction which includes oxygen, peroxide or a combination of the same.  
     
     
         10 . The improvement of  claim 9  wherein said extraction is followed by one or more exposures of the pulp to chlorine dioxide.  
     
     
         11 . The improvement of  claim 1  wherein said step of subjecting of the pulp at a medium consistency for said time period effects substantial removal of hexauronic acid from the pulp.  
     
     
         12 . The improvement of  claim 10  wherein at least 50 and about 80% of the hexauronic acid originally in the pulp is removed.  
     
     
         13 . In a process for preparation of a digested cellulosic pulp for use in a papermaking process, the improvement comprising the steps of subjecting the pulp at a consistency between about 10% and about 15% based on the weight of oven dried pulp in a vessel to chlorine dioxide for a time period of at least 60 minutes, thereafter subjecting this first-treated pulp to a bleaching sequence which includes at least one stage in which the pulp is subjected to chlorine dioxide for a time period sufficient to produce a pulp of a desired brightness and viscosity, and thereafter recovering the pulp for use in a papermaking process.  
     
     
         14 . A sequence for preparation of digested cellulosic pulp for use in papermaking comprising D emc  followed by one or more of either E o , E p , Eop, D 1  or D 2 .  
     
     
         15 . The sequence of  claim 13  wherein said D emc  is followed by E op  and D 1 .  
     
     
         16 . The sequence of  claim 14  and including D 2  following D 1 .  
     
     
         17 . A method for retrofitting a preexisting multi-stage digested cellulosic pulp treatment facility comprising the step of incorporating into said facility a D emc  first stage.  
     
     
         18 . The method of  claim 16  wherein the preexisting multistage facility includes a D o  stage and said D emc  stage supplants said D o  stage.  
     
     
         19 . The method of  claim 17  wherein said preexisting multistage facility includes a D 1  stage and said D emc  stage supplants said D 1  stage.  
     
     
         20 . The method of  claim 17  wherein said preexisting multistage facility includes a D 2  stage and said D emc  stage supplants said D 2  stage.  
     
     
         21 . A method for the removal of hexauronic acid from a digested cellulosic pulp comprising contacting a medium consistency pulp containing hexauronic acid in a vessel with chlorine dioxide for a time sufficient to extract said hexauronic acid from said pulp, said time being not less than about 75 minutes.  
     
     
         22 . The method of  claim 21  wherein said pulp in said vessel is of a pH of between about 2 and about 4 and at a temperature of between about 100 and about 170 degrees F.