Investigation on the bleachability of hemicellulase-deinked pulp versus chemical deinking process

In this study, the ability to bleaching hemicellulose-deinked pulp was investigated in comparison with conventional chemical method. Initially, the mixed recycled pulp pre-treated with hemicellulase enzyme at different levels of usage, treatment time and pH values (under other constant process conditions) was bleached and experimental runs included H2T3 (usage 0.2% (based on dry weight of recycled pulp); Treatment time 60 minutes and pH range 6.5-7) and H3T2 (usage 0.5% (based on dry weight of recycled pulp); Treatment time 50 minutes and pH range 6.5-7) were selected as the best treatments. Then, the selected pulps were bleached using hydrogen peroxide under constant process conditions and their bleachability was compared with conventional chemical pulp. Based on the results achieved, the bleachability of hemicellulose-deinked pulps was better than that of chemically deinked pulp, and the resulting papers had a higher brightness and whiteness as well as less yellowness, but the opacity of these papers was decreased during bleaching. Also, pre-treated pulps with hemicellulase showed the papers with higher tensile and burst indices after bleaching stage so that for H2T3 and H3T2 enzymatic runs, the increase in tensile index was 13.22% and 12.59%, and in burst index was 3.66% and 6.22%, respectively. While the tensile and burst indices for chemically deinked pulp after bleaching was improved by 10.17% and 16%, respectively. Also, the reduction in paper tear index for H2T3 and H3T2 enzymatic runs after bleaching was 12.45% and 8.06%, respectively, compared to chemically deinked pulp (4.44%). Based on the overall results obtained by comparing the quality properties of papers made from different pulps bleached with hydrogen peroxide, H2T3 run has resulted in more bleachability and much better optical and strength properties.