Process of bleaching a wood pulp

Abstract

A process of bleaching a wood pulp includes contacting the pulp with ozone to delignify the pulp in an ozone bleaching stage. A phosphonate chelant is added to the pulp so that the chelant is present with the pulp during the bleaching stage. The process excludes washing of the pulp to remove chelant between the chelant addition and the bleaching stage.

1 . A process of bleaching a wood pulp comprising:
 in an ozone bleaching stage, contacting the wood pulp with ozone to delignify the pulp; and   adding a phosphonate chelant to the pulp so that the chelant is present with the pulp during the bleaching stage;   the process excluding washing of the pulp to remove chelant between the chelant addition and the bleaching stage.   
     
     
         2 . The process of  claim 1  wherein the phosphonate chelant comprises an ethane 1-hydroxy diphosphonate. 
     
     
         3 . The process of  claim 2  wherein the phosphonate chelant comprises a blend of a second phosphonate chelant with the ethane 1-hydroxy diphosphonate. 
     
     
         4 . The process of  claim 2  wherein the phosphonate chelant consists solely of the ethane 1-hydroxy diphosphonate. 
     
     
         5 . The process of  claim 1  wherein the phosphonate chelant is present during the bleaching stage at a concentration within a range of from about 50 ppm to about 12,000 ppm. 
     
     
         6 . The process of  claim 5  wherein the chelant concentration is within a range of from about 500 ppm to about 3,000 ppm. 
     
     
         7 . The process of  claim 1  wherein the ozone charge during the ozone bleaching stage is within a range of from about 0.1% to about 1%. 
     
     
         8 . The process of  claim 7  wherein ozone charge is within a range of from about 0.2% to about 0.8%. 
     
     
         9 . The process of  claim 1  wherein the ozone bleaching stage is conducted at a temperature within a range of from about 50° F. to about 150° F. 
     
     
         10 . The process of  claim 1  wherein the selectivity of the delignification is improved by at least about 10% compared to the same process without the chelant. 
     
     
         11 . The process of  claim 10  wherein the selectivity of the delignification is improved by at least about 30%. 
     
     
         12 . The process of  claim 1  wherein the ozone bleaching stage excludes a peroxide bleaching agent. 
     
     
         13 . The process of  claim 12  wherein the ozone bleaching stage excludes all delignification agents besides the ozone. 
     
     
         14 . The process of  claim 1  wherein the pH of the pulp at the end of the bleaching stage is within a range of from about 1 to about 7. 
     
     
         15 . The process of  claim 1  wherein the ozone bleaching stage is conducted without the use of cyanamide or cyanamide salt. 
     
     
         16 . A process of bleaching a wood pulp comprising:
 in an ozone bleaching stage, contacting the wood pulp with ozone to delignify the pulp, the ozone charge during the ozone bleaching stage being within a range of from about 0.1% to about 1%; and   adding a phosphonate chelant which comprises an ethane 1-hydroxy diphosphonate to the pulp so that the chelant is present with the pulp during the bleaching stage;   the process excluding washing of the pulp to remove chelant between the chelant addition and the bleaching stage;   the process improving the selectivity of the delignification at least about 30% compared to the same process without the addition of the chelant.   
     
     
         17 . The process of  claim 16  wherein the phosphonate chelant consists solely of the ethane 1-hydroxy diphosphonate. 
     
     
         18 . The process of  claim 17  wherein the chelant concentration is within a range of from about 500 ppm to about 3,000 ppm. 
     
     
         19 . The process of  claim 18  wherein ozone charge is within a range of from about 0.2% to about 0.8%. 
     
     
         20 . The process of  claim 19  wherein the ozone bleaching stage is conducted at a temperature within a range of from about 50° F. to about 150° F.