IMPROVED BLEACH PLANT OPERATIONS-TO REDUCE T BLEACH CHEMICALS AND EFFLUENT COLOR

Abstract

The major issues in Indian bleach plant operations are essentially environment related particularly those of chloro organics release and color of the effluent. The study examines the effects of chlorine dioxide substitution, oxygen delignification and ozone delignification on pollutant levels in the bleach plant effluents. Experiments were performed on an unbleached Kraft pulp obtained from The Andhra Pradesh Paper Mills Ltd., Rajahmundry, India. At target brightness of 86+% ISO the pulps have been bleached by C85/D15EpDEpD, D50/C50EpDD, D50/C50EopDD, 0050/D50EpDD, OC90/D1OEpDD, ODEpD, OZEDPD and (PQ)ZEP sequences. The bleach chemical consumption, pulp characteristics viz, brightness and viscosity, and the effluent characteristics expressed as COD, color and AOX have been measured and compared for the above mentioned bleaching sequences. The results indicate that the substitution of chlorine by chlorine dioxide reduces the pollutant levels expressed as color, COD and AOX with improvement in the pulp viscosity. The sequences like ODEpD and OD50/C50EPDD are recommended as sequences that give bleached pulp of desired brightness levels with chemical cost savings of Rs. 590 and Rs. 470 per tonne of bleached pulp, with the reduction in COD of 76% and 61% and color load reduction of 73% and 53% respectively over the C85/D15EpDEpD sequence. The viscosity values are comparable with that obtained after Cgs/D 15EpDEpD sequence. Ozone delignification was performed at low consistency (3%), which shows reduction in the effluent load when compared with Cgs/D15EpDEpD sequence. The mill may seriously consider adopting ECF bleaching sequence with oxygen delignification as a strategy for meeiing economic quality and environment requirements.