Elemental chlorine free bleaching impact on metal management

Abstract

With the implementation of the Cluster Rule in the United States, all bleached grade kraft pulpmills were required to meet stringent levels of absorbable organic halides in the effluent The method most mills used to meet the absorbable organic halide levels was to convert to 100% substitution of chlorine dioxide in the predominately C or CD first stage. As mills substituted chlorine dioxide in the first stage of the bleachplant, thereby converting to elemental chlorine free (ECF) bleaching, several key parameters were impacted. Two of these were that delignification in ECF was less than partial substitution of chlorine dioxide in the chlorine stage and secondly the increase in pH in the first stage did not give the same level of metals solubility as compared to chlorine or CD bleaching. Metals management has become a significant issue in ECF bleaching as metals that previously had a minor influence in bleaching now create issues with inorganic deposition and efficiency reduction of peroxide bleaching. Several operating conditions have been identified that when optimized, reduced the issues associated with scale development in various stages of the bleachplant.