Method for brightening mechanical pulps

Abstract

The present invention is directed to an improved method for brightening mechanical pulp under neutral or alkaline papermaking conditions. The improvement comprises the steps of: (a) separating neutral or alkaline pulp dilution water into a high-solids stream and a neutral or alkaline low-solids stream; and (b) reusing the neutral or alkaline low-solids stream for pulp dilution purposes prior to a bleaching process.

1 . An improved method for brightening mechanical pulp under neutral or alkaline paper making conditions; said improvement comprising steps of: 
 (a) separating neutral or alkaline pulp dilution water into a high-solids stream and a neutral or alkaline low-solids stream; and    (b) reusing the neutral or alkaline low-solids stream for pulp dilution purposes prior to a bleaching process.    
     
     
         2 . The method of  claim 1  in which said high-solids stream is treated with at least one chelant and at least one reducing agent to produce a treated high-solids stream.  
     
     
         3 . The method of  claim 2  in which the neutral or alkaline pulp dilution water and the neutral or alkaline low-solids stream have a pH from 6 to 8.  
     
     
         4 . The method of  claim 3  in which the treated high-solids stream is added to mechanical pulp entering a paper making machine.  
     
     
         5 . The method of  claim 3  in which the treated high-solids stream is introduced into a bleaching step.  
     
     
         6 . The method of  claim 3  in which the pulp dilution water contains suspended calcium carbonate.  
     
     
         7 . The method of  claim 6  in which said at least one reducing agent comprises dithionite anion.  
     
     
         8 . The method of  claim 1  in which the low-solids stream is added to mechanical pulp entering a paper making machine.  
     
     
         9 . The method of  claim 8  in which the neutral or alkaline pulp dilution water and the neutral or alkaline low-solids stream have a pH from 6 to 8.  
     
     
         10 . The method of  claim 1  in which the high-solids stream is added to mechanical pulp entering a paper making machine.