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 microwave heating, compared to 45 min with water-bath heating. The maximum brightness attained with water-bath heating (78%) was nearly one point higher than that obtained with microwave heating, albeit after twice the time (120 min vs. 60 min). Under similar conditions (70°C, 60min, 4.5% H202), a higherbrightness (78.2%) was obtained with a sodium 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 peroxide consumption. The brightness gain (percent) per percent of peroxide consumed was higher at 10% than at 20% consistency. In rapid high-temperature bleaching using microwave heating, excellent results were obtained without applying microwave energy to the pulp for the entire bleaching time by keeping the heated pulp in a thermos.