New approaches to bleaching of industrial kraft pulp

SUMMARY New Approaches to Bleaching of Industrial Kraft Pulp During the conventional pulp bleaching process, which is used chlorine containing chemicals, reaction of chlorine with residual lignin in pulp and formation of toxic chlorinated organic compounds are inevitable. Hazardous bleach plant effluents had been discharged to watershed up to the near past. Environmental concern and tightened public regulations put forward some new improvements on pulp bleaching such as adoption of oxygen delignification prior to bleaching process, ECF (Elemental Chlorine Free) and TCF (Totally Chlorine Free) bleaching technologies etc. In this study, developing of novel bleaching sequences with low-cost and environmentally acceptable was aimed. Unbleached softwood kraft pulp produced in Mopak Dalaman Pulp and Paper Mill was chosen as experimental material. Three different bleaching sequences were studied in laboratory-scale and the results were compared with those of mill-scale bleaching. The pulp was activated with 1% peracetic acid, before the oxygen delignification in the beginning of the bleaching sequence. Then OPa(EOP)PaPD bleaching sequence which is used oxygen based chemicals dominantly was applied with two ways: Pa stage carried out at pH 6±0,5 (neutral Pa stage) and 4±0,5 (acidic Pa stage) respectively. It was determined that OPa(EOP)PaPD bleaching sequence containing neutral Pa stages gave the best delignification, viscosity and brightness values. Additionally, industrial kraft pulp bleached with ODEDED sequence gave the higher pulp viscosity at the same brightness level than those of OPa(EOP)PaPD bleaching process because of selective lignin removal ability of chlorine based bleaching agents. When the findings of mill scale bleaching application (CEHDED) used in Mopak Dalaman Pulp Mill compared with those of ODEDED sequence examined in this study, CEHDED sequence gave the best brightness level but its potential disadvantages such as unacceptable viscosity loss (about 17,72 %), huge amount AOX formation (7 times higher than the world standards) and relatively higher chemical cost must be questioned. Key Words: Bleaching, Brightness, AOX, TCF, ECF, Peracetic acid, Kraft pulp VII