Abstract
Abstract Bleachability is evaluated as how easily a pulp sample is bleached and it depends on the structure of residual lignin and carbohydrates. Also, the bleachability varies depending on the bleaching sequence. ECF light sequences have been improved significantly in the recent years. However, we still don’t fully understand how ECF light bleach plants are optimally run. This work studies the bleachability of softwood kraft pulp in an ECF light bleaching sequence, (OO)Q(OP)D(PO). Three pulp samples with brown stock kappa number 27, 32 and 35 were bleached and studied for residual lignin, hexenuronic acid and carbohydrate content. It was found that in the bleaching stages that are highly delignifying, it is beneficial with a higher kappa number for the delignifying bleachability. However, in the bleaching stages where the objective is brightness increase, the brightness gain bleachability is improved by a lower kappa number. We also intended to determine which of the three samples had the best suited kappa number for this particular bleaching sequence. According to our results, the bleaching was most effective with kappa number around 32. Although an even higher kappa number resulted in higher yield after cooking, it seemed that this bleaching sequence cannot preserve the yield gain.
Highlights
Bleaching of chemical pulp is a process where residual lignin, chromophores and extractives are removed from the brown stock pulp to produce bright and clean products such as printing paper, tissue and packaging board
Instead of aiming at the same kappa number after oxygen delignification for the three pulp samples, the desired degree of delignification was set to 60 % with varying kappa numbers as a result
The delignifying and brightness gain bleachability can be used to evaluate the bleachability of the stages separately in a bleaching sequence, so that it is possible to distinguish the bleaching efficiency in each stage
Summary
Bleaching of chemical pulp is a process where residual lignin, chromophores and extractives are removed from the brown stock pulp to produce bright and clean products such as printing paper, tissue and packaging board. In order to maintain a high selectivity in the bleaching process, with high yield and pulp quality as a result, the bleaching is carried out in a sequence, alternating between bleaching stages. In ECF, the process is based on bleaching with chlorine dioxide of various charges (Björklund et al 2002, Hart and Connell 2006, Lachenal et al 2008). In TCF, the pulp is bleached with chemicals that do not contain any chlorine, but oxygen based chemicals such as oxygen, hydrogen peroxide (Brelid et al 1997, Sjöström 1998), ozone and peracetic acid. ECF light processes combine the efficiency and high selectivity of chlorine dioxide bleaching with more environmentally friendly oxygen based bleaching chemicals. Better understanding of optimal bleach plant operating parameters is needed, in order to minimize the environmental impact and cost while assuring high pulp quality
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
