Aox mitigation in pulp bleaching effluents using elemental chlorine free bleaching

Abstract

The pulping and bleaching are two major processes of prime environmental concern during paper manufacture due to the generation of highly polluted effluents. The bleaching is aimed at removing the residual lignin to achieve desired pulp brightness. Cl2 or chlorine-based bleaching chemicals generate a variety of chlorinated organics, measured collectively as adsorbable organic halides (AOX), in the bleaching effluents. The disposal of bleaching effluents containing AOX poses an enormous threat to the aquatic organisms and public health. The environmental concern has led to the substitution of Cl2 based conventional bleaching with elemental chlorine-free (ECF) and chlorine-free (TCF) bleaching to regulate AOX generation in the effluents. The present study is aimed at reducing AOX generation in the bleaching effluent by using eco-friendly ECF bleaching technology (i.e. replacement of elemental chlorine with ClO2, H2O2 reinforced alkaline extraction and O2 delignification). The mixed hardwood kraft pulp is bleached to 87% ISO target brightness using D/CED, DED, DEPD, and ODED sequences under the controlled laboratory conditions. The ECF bleaching sequences (0.67, 0.55, and 0.34 kg/t OD pulp for DED, DEPD, and ODED, respectively) can reduce AOX generation below the set regulatory standard (1 Kg/T OD pulp) as compared to D/CED sequence (2.6 kg/t OD pulp). ODED sequence saved 45% bleach chemical as compared to the D/CED sequence. The ECF bleaching sequences can cut NaOH dose by 30% during the alkaline extraction (E/EP) stage as compared to D/CED sequence. ODED and DEPD bleaching sequences are found appropriate to meet the AOX discharge standard.