Alkaline peroxide treatment of rejects in an intergrated neutral-alkaline paper mill

Abstract

A processes producing mechanical pulps utilizing refiners for improving the quality of screen rejects, and most particularly to a mechanical pulp mill integrated with a neutral-alkaline papermaking process producing mechanical printing paper grades is disclosed. The rejects are treated with hydrogen peroxide, an alkali and an organic stabilizing agent just prior to or during refining to provide improved optical and physical properties of the refined rejects and more efficiently utilize electrical energy to reach target fiber quality after bleaching for cost-effectively producing a wide array of coated and uncoated mechanical printing papers.

1 . A method for treating mechanical pulps comprising the steps of
 a) providing cellulosic materials derived from softwood or hardwood;   b) introducing the cellulosic material to a mainline refining system for conversion to a mainline pulp;   c) separating out rejects from the mainline pulp by means of screening or fractionation;   d) mixing a neutral-alkaline whitewater stream, recycled from a paper process, essentially free of oxidizable sulfur species and containing a buffering agent, with said rejects to form a rejects fiber;   e) adding a treatment liquor comprising hydrogen peroxide and an alkali to said rejects fiber, wherein the amount of alkali is less than about 3.5% by weight (100% basis) based on fiber and the ratio of alkali to hydrogen peroxide is from about 0.35/1.0 to about 1.5/1.0; and   f) adding a chelant to control metal ions harmful to hydrogen peroxide stability to said rejects fiber and said neutral-alkaline whitewater stream.   
     
     
         2 . The method of  claim 1 , wherein said alkali is selected from the group consisting of sodium hydroxide, sodium carbonate or sodium bicarbonate. 
     
     
         3 . The method of  claim 1  wherein said chelant is selected from the group consisting of aminopolycarboxylic acids, phosphonic acids, polycarboxylic acids, polyacrylates, polyaspartates, gluconates or citrates. 
     
     
         4 . The method of  claim 1  wherein said alkali and hydrogen peroxide are added in combination with said rejects fiber ahead of refining. 
     
     
         5 . The method of  claim 1  wherein said buffering agent is selected from the group consisting of precipitated calcium carbonate (PCC) or ground calcium carbonate. 
     
     
         6 . The method of  claim 1  wherein said alkali and hydrogen peroxide are added in combination with said rejects fiber during refining.