Operating Experience with the High-Consistency Bleaching System in the RTS - Plant at Stora Enso, Port Hawkesbury, 1999 International Mechanical Pulping Conference Proceedings

Seldon MacKenzie & Rita Ferritsius

Stora Enso

Peter Bräuer, Peter Kittel & Mark Frith

Andritz AG

In April 1998 the high-consistency bleaching system at the new RTS TMP plant at Stora Enso Port Hawkesbury, Nova Scotia (Canada) was started-up.

The pulp, which is produced from a furnish of various northern softwoods, is bleached to brightness levels up to 77% ISO with a moderate amount of peroxide is used for the production of SC-A paper grades on the new paper machine. This paper shortly describes the plant configuration and the key equipment used, and provides an overview of the most important operating conditions. Due to the extremely low freeness of the pulp of < 30="" ml="" csf,="" dewatering="" is="" a="" key="" issue="" in="" the="" plant.="" twin="" wire="" presses,="" which="" are="" able="" to="" reach="" high="" capacities="" together="" with="" high="" drynesses="" even="" at="" this="" low="" freeness="" range="" are="" used="" ahead="" of="" the="" high-consistency="" bleach="" tower="" and="" after="" bleaching,="" to="" remove="" the="" detrimental="" substances="" ahead="" of="" the="" paper="">

Since the wood has a substantial manganese content it is necessary to add a sufficient amount of complexing agent in the latency chest to keep the manganese content in the pulp below a critical level and to achieve optimum bleaching results.

Special attention was paid to the optimum design of the water loops to achieve excellent pulp quality at a minimum fresh water and chemicals consumption. By recirculation of the filtrate of the wash press residual peroxide and bleached fines are kept in the TMP system and help improve the unbleached brightness of the pulp.

The results of a mill study, which was performed during the start-up and optimisation period demonstrate that the washing stage after peroxide bleaching removes 85% of the detrimental substances ahead of the paper machine. Plant data is presented, showing the influence of bleaching on strength and optical properties.