The Effect of Different Charges of Sodium Dithionite and Bleaching Times on Optical, Mechanical and COD Properties of Bleached Mixed ONP and OMG Pulp

This research was accomplished to investigate the bleaching of combined old newspapers (ONP) and old magazines (OMG) deinked by sodium dithionite. The bleaching of pulp was performed using a 0.5, 0.75, and 1% sodium dithionite and different times of 30, 60 and 90 min. The pulps were refined up to freeness of 300 ml, CSF and standard handsheets made from them. The optical and mechanical properties of paper and COD of bleaching effluent were measured according to TAPPI standard methods.The results showed that the brightness of paper was increased significantly with increase in the time and sodium dithionite charge. When the charge of bleaching agent increased, the mechanical properties of paper and COD pollution load of bleaching effluent were increased and paper bulk and opacity decreased but these changes were not statistically significant. Increasing in bleaching time from 30 to 90 min resulted in a significant improvement in tensile, burst and tear indices and decrease the opacity of paper.