Method of treating chemical cellulose pulp

Abstract

A method of bleaching chemical cellulose pulp from an alkaline (e.g. kraft) pulping process in a chlorine dioxide stage includes (without between step washing) bleaching the pulp in a first chlorine dioxide step, and adjusting the pH of the pulp in the first chlorine dioxide step so that the final pH of the step is over 4; and then (b) effecting acid treatment of the chemical cellulose pulp at a pH of between 2-5 (preferably 2.5-4) and a temperature of over 80° C. (preferably between about 90-110° C.). The temperature in the first chlorine dioxide stage is preferably over 75° C., e.g. between about 80-100° C., and for a time of less than ten minutes, with a chlorine dioxide dosage of between about 0.5-1.5% active chlorine. Preferably a second chlorine dioxide step is practiced after the acid treatment, preferably at substantially the same conditions as the first chlorine dioxide step. The method minimizes the consumption of chlorine dioxide to bleach to a particular kappa number, minimizes the amount of equipment, enhances energy economy of the chlorine dioxide bleaching process, and minimizes the discharge of malodorous gases since they are oxidized by the first chlorine dioxide step.

What is claimed is:  
     
         1 . A method of treating chemical cellulose pulp from an alkaline pulping process in a chlorine dioxide stage comprising: 
 (a) in the chlorine dioxide stage, bleaching the chemical cellulose pulp in a first chlorine dioxide step, and adjusting the pH of the pulp in the first chlorine dioxide step so that the final pH of the step is over 4; and then    (b) in the chlorine dioxide stage effecting an acid treatment of the chemical cellulose pulp at a pH of between 2-5 and at a temperature of over 80° C.    
     
     
         2 . A method as recited in  claim 1  wherein (a) is practiced so that the final pH of the first chlorine dioxide step is over 5, and so that hexenuronic acid groups in the pulp substantially do not react with chlorine dioxide.  
     
     
         3 . A method as recited in  claim 2  wherein (a) is further practiced so that the temperature in the first chlorine dioxide step is over 75° C.  
     
     
         4 . A method as recited in  claim 2  wherein (a) is further practiced so that the temperature in the first chlorine dioxide step is between about 80-100° C.  
     
     
         5 . A method as recited in  claim 4  wherein (a) is further practiced so that the treatment time in the first chlorine dioxide step is less than 10 minutes.  
     
     
         6 . A method as recited in  claim 4  wherein (a) is further practiced so that the treatment time in the first chlorine dioxide step is between 30 seconds-3 minutes.  
     
     
         7 . A method as recited in  claim 5  further comprising (c) bleaching the chemical cellulose pulp, after (b), in a second chlorine dioxide step.  
     
     
         8 . A method as recited in  claim 7  wherein (a)-(c) are practiced so that the treatment temperatures in the first chlorine dioxide step, the acid treatment step, and the second chlorine dioxide step, substantially the same.  
     
     
         9 . A method as recited in  claim 1  further comprising (c) treating the chemical cellulose pulp with chelating agent after (a) and (b).  
     
     
         10 . A method as recited in  claim 1  wherein (a) is further practiced so that the temperature in the first chlorine dioxide step is over 75° C.  
     
     
         11 . A method as recited in  claim 1  wherein (a) is further practiced so that the temperature in the first chlorine dioxide step is between about 80-100°C.  
     
     
         12 . A method as recited in  claim 1  wherein (a) is further practiced so that the treatment time in the first chlorine dioxide step is less than 10 minutes.  
     
     
         13 . A method as recited in  claim 1  wherein (a) is further practiced so that the treatment time in the first chlorine dioxide step is between 30 seconds-3 minutes.  
     
     
         14 . A method as recited in  claim 1  further comprising (c) bleaching the chemical cellulose pulp, after (b), in a second chlorine dioxide step.  
     
     
         15 . A method as recited in  claim 14  wherein (a)-(c) are practiced so that the treatment temperatures in the first chlorine dioxide step, the acid treatment step, and the second chlorine dioxide step, substantially the same.  
     
     
         16 . A method as recited in  claim 4  wherein (a) is further practiced so as to provide a chlorine dioxide dosage of between about 0.5-1.5% active chlorine during the first chlorine dioxide step.  
     
     
         17 . A method as recited in  claim 7  wherein (a) is further practiced so as to provide a chlorine dioxide dosage of between about 0.5-1.5% active chlorine during the first chlorine dioxide step; and wherein (c) is practiced so as to provide a chlorine dioxide dosage of between about 0.5-2.0% active chlorine during the practice of the second chlorine dioxide step.  
     
     
         18 . A method as recited in  claim 17  wherein step (b) is practiced at a pH between 2.5-4, a temperature between 90-110° C., and a time between 30-300 minutes.  
     
     
         19 . A method as recited in  claim 18  wherein (a)-(c) are practiced so that the treatment temperatures in the first chlorine dioxide step, the acid treatment step, and the second chlorine dioxide step, are substantially the same, and between about 90-100° C.  
     
     
         20 . A method as recited in  claim 19  wherein (a) through (c) are practiced utilizing an acid tower, an inlet line to the acid tower, and an outlet line from the acid tower to a further treatment device; and wherein (a) is practiced substantially completely within the inlet line to the acid tower, (b) is practiced substantially completely within the acid tower, and (c) is practiced substantially completely in the discharge line from the acid tower.