Measurements In Chlorine Dioxide (ClO2) Bleaching Used In Pulp Mills

Bleaching is a whitening process that is used in the paper industry to produce paper with high brightness. Chemicals—such as chlorine gas (C), sodium hypochlorite (H), oxygen (O), hydrogen peroxide (X), ozone (Z), and chlorine dioxide (D)— are used in various combinations to produce pulp with the desired properties. Recent limitations on the use of chlorine (and hypochlorite) have resulted in more extensive use of chlorine dioxide as an alternate bleaching chemical. Chlorine dioxide bleaching is referred to as Elemental Chlorine Free (ECF) and is the process used in over 50% of U.S. mills.

The process
The chlorine dioxide is mixed with the pulp and contacted in each D (chlorine dioxide) tower (see Figure 1) for about 1 hour per stage. The primary purpose of bleaching is to maximize removal of the lignin binding agents in the pulp without degradation of carbohydrates, which weakens the final product (paper). The pH is maintained at relatively low levels (around 4) to foster rapid chemical reaction. Higher pH levels require more chlorine dioxide for equivalent bleaching action.

Following bleaching in each D tower, the reacted lignin is removed by dissolving the lignin under high pH conditions (typically over 11). Lower pH levels will not dissolve all of the lignin. This caustic extraction (E) stage is preceded by a washing step to reclaim the chlorine dioxide and to minimize carryover of the acidic chemicals. The amount of caustic needed is proportional to the amount of bleach added in the D stage.

Bleach plant sequences vary widely from mill to mill; however, the D-E-D-E stage sequence is quite common. At least 2 bleaching stages are generally required to produce the specified brightness, with additional stages for more specialized uses. Consistency (% solids) is typically maintained at a high level (up to 10%) to minimize water use. Temperature is controlled around 140°F (60°C), and doses of 5% chlorine dioxide are common.