Method for bleaching wood fibers

Abstract

A method for bleaching wood fibers with a mixture of sodium borohydride and sodium bisulfite followed by peroxide bleaching in the same vessel.

1 . A method for bleaching wood fibers; said method comprising steps of: 
 (a) adding to a vessel containing a wood fiber slurry a mixture produced by combining: (i) an aqueous solution comprising sodium borohydride and sodium hydroxide; and (ii) an aqueous solution comprising sodium bisulfite, to produce a treated slurry; and    (b) adding an aqueous solution comprising hydrogen peroxide to the treated slurry in said vessel.    
   
   
       2 . The method of  claim 1  in which the aqueous solution comprising hydrogen peroxide is added from 5 seconds to 2 hours after said mixture.  
   
   
       3 . The method of  claim 2  in which an amount of sodium borohydride used to produce said mixture is from 0.0006% to 0.045%, based on the weight of dry fiber.  
   
   
       4 . The method of  claim 3  in which a ratio of (moles bisulfite-moles hydroxide)/moles borohydride combined to produce said mixture is from 1 to 25.  
   
   
       5 . The method of  claim 4  in which an amount of sodium bisulfite added is from 0.02% to 1.5%.  
   
   
       6 . The method of  claim 5  in which the wood fiber slurry is derived from recycled paper.  
   
   
       7 . The method of  claim 6  in which the aqueous solution comprising hydrogen peroxide is added from 5 seconds to 15 minutes after said mixture.  
   
   
       8 . The method of  claim 7  in which a ratio of (moles bisulfite-moles hydroxide)/moles borohydride combined to produce said mixture is from 6 to 10.