PEROXIDE BRIGHTENING OF MECHANICAL PULPS

摘要

A method of brightening mechanical pulps by chelating and thickening to reduce the manganese content of the pulp to less than 30 parts per million (Cu less than 1 parts per million) adding ions preferably magnesium ions (e.g. magnesium sulphate MgSO4,) in the amount of 400 to 3,000 ppm Mg ions retained by the pulp based on the oven dry weight of the pulp and thereafter applying a bleaching liquor composed of peroxide (e.g. hydrogen peroxide) and an alkali (e.g. sodium hydroxide) having a ratio of alkali to peroxide of between 1-1 and 2-1 in an amount to apply between 1 to 8 percent peroxide based on the oven dry weight of the pulp and containing n o added sodium silicate. Preferably the bleaching liquor will contain magnesiu m sulphate in the amount of about 0.01 to 0.1% based on the oven dry weight of the pulp. The treated pulp is bleached in a bleaching tower for a sufficient time to obtain the desired degree of brightening and thereby produce a pulp having a brightness at least equivalent to that obtained by bleaching using a conventional silicate stabilized bleaching liquor and the equivalent amount of peroxide.