Process of chlorine dioxide bleaching of chemical pulps using magnesium hydroxide or magnesium oxide

Abstract

The present invention provides an improved process for chlorine dioxide bleaching of chemical pulps in an aqueous suspension, which uses magnesium hydroxide or magnesium oxide. The process involves the addition of magnesium hydroxide or magnesium oxide to the pulp, in addition to chlorine dioxide, maintaining the mixture at a pH between about 2.5 to 7.5 for about 1 or more hours, most suitably between about 1-4 hours. This process can be used in the D 1 or D 2 stage of a typical D 0 E op D 1 E 2 D 2 bleaching sequence, as well as in the chlorine dioxide brightening stage of other three, four, five, and six-stage bleaching sequences of either the so-called ECF process, or those containing elemental chlorine (Cl 2 ). The temperature is between about 40 to about 90° C., and the pulp consistency is between about 1 to about 16%.

1 . A process for bleaching chemical pulp of the type characterized by a D 0 E op D 1 E 2 D 2  bleaching sequence in which
 D 0 : chlorine dioxide delignification, with the pH controlled in a range from about 2 to about 3;   E op : alkaline extraction with caustic soda, pH is in the range of 10-11.5;   D 1 : first chlorine dioxide brightening stage with pH controlled in a range from about 2.5 to about 7.5;   E 2 : the second alkaline extraction, the end pH is in the range from about 9.5 to about 11.5;   D 2 : the second chlorine dioxide brightening stage, the end pH controlled in a range from about 2.5 to about 7.5, wherein the process comprises the steps of:
 bleaching chemical pulps in the D 1  and/or D 2  stages using chlorine dioxide, including adding chlorine dioxide and magnesium hydroxide or magnesium oxide in the absence of oxygen to a suspension of the chemical pulp to form a mixture, wherein the chlorine dioxide and magnesium hydroxide are added in a sequential order or simultaneously, including maintaining the mixture at a pH between about 2.5 to about 7.5 for about 1 or more hours, and maintaining the mixture at a temperature in a range from about 40° C. to about 90° C., and maintaining a pulp consistency in a range between about 1 to about 16% weight percentage on dried mass of pulp. 
   
     
     
         2 . The process according to  claim 1  wherein said temperature is maintained in a range from about 55° C. to about 80° C. 
     
     
         3 . The process according to  claim 1  including maintaining the mixture at a pH between about 3 to about 7 for between about 1 to about 4 hours. 
     
     
         4 . The process according to  claim 1  wherein said pH of the mixture is maintained in a range from about 3 to about 6. 
     
     
         5 . The process according to  claim 1 , wherein said pulp consistency is maintained between about 3 to about 12%. 
     
     
         6 . The process according to  claim 2  including maintaining the mixture at a pH between about 3 to about 7 for between about 1 to about 4 hours. 
     
     
         7 . The process according to  claim 2  wherein said pH of the mixture is maintained in a range from about 3 to about 6. 
     
     
         8 . The process according to  claim 2  wherein said pulp consistency is maintained between about 3 to about 12%. 
     
     
         9 . The process according to  claim 3  wherein said pulp consistency is maintained between about 3 to about 12%. 
     
     
         10 . The process according to  claim 4  wherein said pulp consistency is maintained between about 3 to about 12%.