Study of bleaching of old newsprint recycled paper: reproduction of newspaper material

Abstract

With the utilization of used newspaper into the paper making industry, bleaching has been an area of concern to obtain a balance between desired brightness and the amount of treatment it requires. Motivation of this study was to optimize bleaching of recycled newsprint fibers in terms of five variables namely hydrogen peroxide charge, amount of sodium hydroxide employed, pulp consistency, bleaching temperature, and bleaching time. Experimental work was conducted in the laboratory to simulate bleaching plant and it was found that higher peroxide charge is better for improvement in brightness, but it is uneconomical to increase peroxide charge indefinitely. The amount of hydrogen peroxide is added so that the residual peroxide at the end of the reaction is not less than 10% of the initially added peroxide. Sodium hydroxide provides alkalinity which is very important for making the fibers flexible and softening of ink particles but it also contributes to pulp yellowing if residual hydrogen peroxide contents are lesser than the minimum required value. So, for 16 kg t−1 of hydrogen peroxide, 6 kg t−1 of sodium hydroxide prevented the brightness reversion in this study. Higher pulp consistency is better but unavailability of very high consistency pulp mixer in the industry is a limiting factor for higher than 30% pulp consistency. Temperature and time are quite interrelated and, with the given bleaching conditions, a temperature of 95 °C with 10 min residence time was found to be optimum for this study. It was also observed that multiple pre-washings increase brightness and save bleaching chemicals.