Conversion to ECF bleaching-A comparative study with focus on the first bleaching stage

Abstract

In the present paper, an investigation of some different ECF bleaching sequence is presented, and the results compared. Special attention is paid to the importance of the first bleaching stage after oxygen delignification, i.e. the so-called delignifying bleaching stage. Two different mill-oxygen-delignified HW kraft pulps were subjected to a number of different bleaching sequences. The first pulp was bleached according to D (EO) D, D * (EO) D, (ZD) (EO) D, Z (EO) D and after interpolation evaluated at 89% ISO brightness, and the second pulp according to D (EP) D, D * (EP) D, (ZD) (EP) D, A * ZD (EP) D and after interpolation evaluated at 87% ISO brightness. The "*" here denotes a treatment at high temperature and log retention time. Both the use of D * and Z efficiently reduced the yellowing tendency of the pulps when compared to the conventional D-based ECF sequences. Based on the laboratory trials, consumption figures were adjusted to reflect mill operating conditions and the differences in operational costs between the different sequences were calculated. The sequences with D * were economically very attractive, showing high savings in operational costs compared to the reference D (EO) D and D (EP) D sequences, especially when the target brightness was somewhat higher, i. e. 89%. Considering the unit costs, the required investments needed for Z-based sequences were found to be difficult to justify, especially when the target brightness is somewhat lower, e.g. 87% ISO.