Peroxide bleaching of mechanical pulps

摘要

A process for peroxide bleaching of mechanical pulps by washing the pulp to remove substantially all of the heavy metal ions from the pulp, mixing hydrogen peroxide in an amount of at least about 11/2% based on the dry weight of the pulp with the washed pulp to uniformly disperse the hydrogen peroxide throughout the pulp followed by mixing an aqueous solution of sodium hydroxide with the pulp containing hydrogen peroxide to uniformly distribute the sodium hydroxide throughout the pulp and to react the sodium hydroxide with the peroxide to produce perhydroxyl ions in situ in the pulp and then reacting the perhydroxyl ions with the pulp to increase the brightness of the pulp and provide a residual peroxide of at least 35% of the peroxide added. This system provides a significant increase in residual brightening chemical in the residual liquor separated from the brightened pulp. In a preferred process to utilize this residual brightening chemicals, the washed pulp is divided into two portions and the hydrogen peroxide is added to one portion as above described, following reaction of the perhydroxyl ions with the pulp of the one portion the so brightened pulp portion is separated from a residual liquor containing residual brightening chemnicals. This residual liquor containing the residual brightening chemicals is uniformly mixed with the second portion of the pulp followed by dispersion of caustic through the second portion to cause reaction of the residual brightening chemicals with the second portion of the pulp to brighten the second portion of the pulp.