POST BLEACHING OF DEINKED PULP FOR NEWSPRINT PRODUCTION

As part of a study into deinking wastepaper for future newsprint production, the post bleaching of deinked pulp has been studied. Deinked pulp was produced in the laboratory from a news/magazine mix, using a typical procedure including hydrogen peroxide, sodium hydroxide, DTPA, sodium silicate and a surfactant The deinked pulp was post bleached using hydrogen peroxide, sodium hydrosulfite, calcium hypochlorite or formamidine sulfinic acid (FAS). For each bleaching agent, the additional chemicals and bleaching conditions were optimized. On an applied chemical basis, FAS is the most effective, then hydrogen peroxide and hydrosulfite, The cost of reagents must be considered in making the final process selection, Post bleaching with calcium hypochlorite darkened the pulp. For hydrogen peroxide bleaching, the relative merits of applying peroxide in the pulper or in post bleaching were compared. With a 2% total charge of hydrogen peroxide shared in different ratios between the pulper and post bleaching, the achieved brightness improvements were similar except when all the hydrogen peroxide was placed in the pulper, to give a brightness nearly one ISO unit greater than other combinations.