Bleachability of alkaline and neutral medium deinked waste paper stocks

摘要

The bleach response of recycled fibres depends on the wastepaper grades, type of printing and deinking processes, chemical composition of the bleaching agents, being obviously determined by the process parameters. The paper investigates the efficiency of three lignin retaining chlorine-free bleaching chemicals (sodium hydrosulphite, formamidine sulphinic acid and hydrogen peroxide) on both wood-free and wood-containing waste paper stocks, deinked on a pilot plant in alkaline or neutral systems. The effectiveness of deinking and bleaching steps in terms of kappa number yield, metal content, COD charge, chemical consumption, as well as ash content, optical and strength properties, has been evaluated. It was found that, in the case of the recovered papers taken into study (ONP/OMG and xerox-print)-in medium consistency processing (of 10%) which avoids air mixing into the stock -, the best results in single-stage treatments with FAS bleaching were obtained, concerning both the brightness level and the COD charge of process water. In multi-stage sequences (for example, AP FAS, or P_1 P_2 FAS) higher brightness can result, alongwith an increased yield losses (6 to 8%) and elevated COD charges (12 to 21 kg O_2/t), and simultaneously with a slight decrease of the strength characteristics of pulp. As to the UV brightness stability of the paper samples, good results with wood-free stocks (xerox and MOW) in neutral deinking were registered.