CHARACTERISTICS OF PEROXIDE BLEACHING OF MICROWAVE-HEATED THERMOMECHANICAL PULPS

A brightness of 76% (initially 57%) was reached with a charge of 4.5% hydrogen peroxide based on thermomechanical pulp in 10 min with microw ave heating, compared to 45 min with water-bath heating. The maximum b rightness attained with water-bath heating (78%) was nearly one point higher than that obtained with microwave heating, albeit after twice t he time (120 min vs. 60 min). Under similar conditions (70-degrees-C, 60 min, 4.5% H2O2), a higher brightness (78.2%) was obtained with a so dium hydroxide-to-hydrogen peroxide ratio of 0.97 compared to that (76 .5%) obtained when a sodium hydroxide-to-hydrogen peroxide ratio of 0. 73 was used. Pulp brightness did not increase proportionately with per oxide consumption. The brightness gain (percent) per percent of peroxi de consumed was higher at 10% than at 20% consistency. In rapid high-t emperature bleaching using microwave heating, excellent results were o btained without applying microwave energy to the pulp for the entire b leaching time by keeping the heated pulp in a thermos.