Improving Chlorine Dioxide Bleaching of a Softwood Kraft Pulp by Using Magnesium Hydroxide for pH Control

Weak alkalis such as magnesium hydroxide may be better alternatives for pH control in chlorine dioxide bleaching by providing a relatively stable pH profile in the whole bleaching process. Results show that substituting Mg(OH)(2) for NaOH in a chlorine dioxide brightening stage for pH control can lead to improved performance of chlorine dioxide in terms of pulp brightness and viscosity due to the buffering effect of Mg(OH)(2). With Mg(OH)(2) as the pH control agent in the first chlorine dioxide brightening stage (D-1), higher pulp brightness can be achieved at the same chlorine dioxide dosage, or the chlorine dioxide dosage can be decreased by up to 30% for a given brightness target of the final bleached pulp. The viscosities of the D-1 pulp and the final bleached pulp were also higher when Mg(OH)(2) was used to replace NaOH for the pH control in the D-1 stage. Mg(OH)(2) can also be used in the second chlorine dioxide brightening stage (D-2) with similar improvement in pulp brightness and viscosity. The buffering effect of Mg(OH)(2) can decrease the alkaline decomposition of chlorine dioxide encountered in a typical chlorine dioxide brightening stage, thus decreasing the formation of chlorate and chlorite. As a result, the bleaching efficiency was improved in the Mg(OH)(2)-based chlorine dioxide brightening process. An additional benefit of using magnesium hydroxide is the increase of Mg2+ ions in the water system of the bleach plant, which can decrease the oxalate-related scaling.