Features Research & Innovation
Further refining the mechanical pulping process

THE MECHANICAL PULPING Committee wrapped up PaperWeek with its two sessions held traditionally on Thursday. In the afternoon session, five papers were presented: three from the halls of academia, one from the Alberta Research Council (ARC) and one from a supplier, Andritz. The three university-based papers dealt with aspects of bleaching mechanical pulp. The session was chaired by Irene Yaraskavitch, Dow Chemical, chair of the committee.

The first paper came from the University of New Brunswick (UNB), Oxygen formation during peroxide bleaching of a thermomechanical pulp and its relationship to bleaching performance. It was authored by W. Yang and Y. Ni. Oxygen formation during peroxide bleaching of TMP was measured. It was discovered that the loss of H2O2 from both the alkali and the transition metal-induced peroxide decomposition can be accounted for by the formation of oxygen.

Based on the amount of oxygen formed, the authors were able to calculate H2O2 consumption in both the decomposition and the pulp oxidation. The H2O2 is consumed continuously in these two competitive reactions. For some chelated pulps, up to 48% of total hydrogen peroxide use may be wasted, but in a well-chelated pulp, only six to 13% of the H2O2 will be consumed in decomposition reactions.

The second paper of the session was authored by C. Leduc, M.M. Sain and C. Daneault of the Universite du Quebec a Trois-Rivieres. It dealt with the use of oxidizing agents such as peroxide, activated peroxide and sodium perborate for the bleaching of mechanical pulp. Two possibilities for adding the oxidizing agents were studies: conventional tower and the eye of the refiner. If sodium perborate is added into the eye of the refiner, it can be a powerful bleaching agent, adding up to 20 points ISO with improvements in strength properties.

The use of an activator — TAED — was also studied. If used in a conventional process, it can improve brightness and allow a reduction in temperature. However, when used in-refiner, it can have negative connotations.

The third university-based paper also came from UNB and was authored by H.C. Nichol and Ni. The authors studied Brightness stabilization of bleached mechanical pulp. The authors observed that thioglycerol (TG) and DTPA was the best combination of inhibitors to combat thermal yellowing. TG is the best single component inhibitor of thermal yellowing because of its radical scavenger behavior. Still, the combination works better than any single inhibitor.

B. Fuhr, ARC, presented a paper, The characterization of anionic trash in the mechanical pulping process, co-authored by R.S. Chow and D. Henry, also from ARC. Anionic trash is material in the white water of mechanical pulp streams composed of organic and inorganic matter. The Millar Western mill in Whitecourt, AB, was used for the study. The study looked at the physical and chemical characteristics of the trash, its distribution in the process and its effect on mill processes such as bleaching and white water treatment.

The distribution of the anionic trash in the process correlates to final pulp brightness. Pulp grades with a brightness of 80 or higher showed a higher cationic demand in the bleach plant relative to those prior to bleaching.

Eric Xu, Andritz, authored the final paper of the session, P-RC alkaline peroxide mechanical pulping of hardwood: Part I – aspen, beech, birch, cottonwood and maple. The species were studied for their pulp property development based on handsheets. The study conducted at Andritz’ R&D facilities showed that the P-RC (preconditioning followed by refiner chemical treatment) alkaline peroxide mechanical pulping process can provide a pulp with good strength and brightness properties (for the hardwoods studied).