Bleaching of chemithermomechanical pulp (CTMP) using environmentally friendly chemicals

摘要

Increasing environmental restrictions require a reduction in the use of chlorine-containing bleaching agents mainly chlorine gas. There are several approaches to this goal, including changes in the cooking process and the influent treatment. The aim of this study is to improve the physical and optical properties of chemithermomechanical pulp (CTMP) which has low brightness and strong tendency towards aging in chlorine-free bleaching sequences. For this reason, initially 7 different sets of single stage hydrogen peroxide bleaching operations were performed. After the analysis of the data obtained, the optimum bleaching conditions were as: H_2O_2 ratio on oven dry (o.d) raw material: 2%, NaOH ratio (on o.d raw material): 1.5%, MgSO_4 ratio (on o.d raw material) : 0.5%, Na_2SiO_3 ratio (on o.d raw material) : 3%, EDTA ratio (on o.d raw material) : 0.5%, reaction time: 60 minutes, reaction temperature: 70℃ and concentration: 16%. Then, 11 different sets of two and multistage H_2O_2 (P), NaBH_4 (B), and Na_2S_2O_4 (S) bleaching operations were carried out. Optimum bleaching conditions were found in bleaching of BPSP with 4 stage sequences. As a result, CTMP brightness was observed to increase from 47% to 81.37% elrepho while yellowness of bleached pulps decreased from 321 per thousand to 164.01 per thousand elrepho.