Progress in bleaching pulps with the sulfite-air system

The effects of pulp pretreatment. pulp consistency. and quantity of sulfite added on final pulp brightness were studied using the cupric ion catalyzed sodium sulfite-air bleaching system. Aspen kraft pulp that had been partially delignified by chlorination and alkaline extraction was used. Highest brightnesses were attained by pretreating the pulp with cupric ion and by using the highest practical consistency. Brightness increased. but at a decreasing rate, with increases in added sulfite. A search was made for other catalysts. Nickelous ion vas found to be effective. but not quite as good as cupric ion. Spruce and aspen kraft pulps. partially delignified by an oxygen treatment. were easily brightened to semibleached levels using the cupric ion catalyzed sodium sulfite-air system. These results indicate that a chlorine-free bleaching system may be feasible.