Combining Chlorine Dioxide Bleaching of Birch Kraft Pulp With an A-Stage at High Temperatures, 1999 Pulping Conference Proceedings

Sari Juutilainen & Tapani Vuorinen

Helsinki University of Technology

Department of Forest Products Technology

Aki Vilpponen, Kaj Henricson & Olavi Pikka

Ahlström Machinery Corporation

D/A/D and A/D prebleaching stages of an oxygen bleached kraft pulp were studied at a high temperature (95 o C) and compared with a conventional Do stage at 75 o C. The initial pH in the D/A/D stage was high (pH 8.5-10) in order to prevent formation of hypochlorous acid (from chlorite ion) and oxidation of HexA. The final bleaching was performed with an EoDnD sequence.

The initial pH of the D/A/D sequence was decreased in a few seconds to a constant level of 5.3-7.0 (depending on initial pH) indicating a complete disappearance of chlorine dioxide. This interpretation was supported by a simultaneous disappearance of the yellow color of the bleaching liquor. At this point the concentration of titrable active chlorine was still high, in accordance with a low reactivity of chlorite ion at high pH. Liberation of 2-furoic acid during the following acid treatment showed that little HexA was degraded by the initially charged chlorine dioxide. The A/D prebleaching was studied by varying the reaction time after addition of chlorine dioxide. The shortest reaction time applied (2 min) led to the highest brightness and viscosity whereas the kappa number was slightly reduced with the longer reaction times.

A comparision at 90 % ISO brightness showed that the viscosity of the pulp in the sequences with the acid treatment was ~65 dm 3 /kg lower than in the reference bleaching. There was, however, no difference in yield and strength properties between the sequences. Savings of about 10 kg ClO2 as active chlorine/BDMT was obtained in the total chlorine dioxide consumption, when D/A/D or A/D prebleaching was used. For a high brightness, it was better to apply a short retention time at high temperature (95 o C, 2 min) in the D-stage (A/D) rather than a longer one at a lower temperature (75 o C, 60 min). The higher temperature did not decrease the viscosity of the pulp.