PROCESS OF CHLORINE DIOXIDE BLEACHING OF CHEMICAL PULPS USING MAGNESIUM HYDROXIDE OR MAGNESIUM OXIDE
The present invention provides an improved process for chlorine dioxide bleaching of chemical pulps in an aqueous suspension, which uses magnesium hydroxide or magnesium oxide. The process involves the addition of magnesium hydroxide or magnesium oxide to the pulp, in addition to chlorine dioxide, maintaining the mixture at a pH between about 2.5 to 7.5 for about 1 or more hours, most suitably between about 1 - 4 hours. This process can be used in the D1 or D2 stage of a typical D0EopD1E2D2 bleaching sequence, as well as in the chlorine dioxide brightening stage of other three, four, five, and six-stage bleaching sequences of either the so-called ECF process, or those containing elemental chlorine (Cl2). The temperature is between about 40 to about 90°C, and the pulp consistency is between about 1 to about 16%.