The effect of different charges of sodium dithionite and bleaching times on the optical and mechanical properties and COD of bleached recycled mixed ONP and OMG pulps

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