Evaluation of Bleach Plant Partial Closure, Part Two: A North American ECF Kraft Mill With Oxygen Delignification, 1998 Pulping Conference Proceedings

Partial Closure Plus (PC+) is Eka Chemicals’ concept of recycling bleach plant alkaline filtrate to the recovery cycle in order to reduce AOX, COD, and color emissions from the bleach plant. Alkaline filtrate is concentrated using a membrane separation process and recycled to the brownstock fiberline as washwater. This concentrated alkaline filtrate is washed back to the recovery boiler where the AOX, COD, and color it contains is destroyed.

Bleach plant acid filtrate is rich in chloride and nonprocess elements so it is sent to the mill’s effluent treatment system as usual and acts as a system purge. Typically, 75% of the bleach plant’s chlorides can be purged with this stream.

A large sampling program was undertaken at Georgia-Pacific’s Leaf River pulp mill in 1997 to serve as the base case for evaluating partial closure plus within the mill by computer simulation. Chemical analyses were performed to provide complete material balances for pulp, water, sodium, sulphur, solids, chloride, potassium, AOX, and COD. This work is a continuation of work done in 1996 on the Leaf River mill (1). The original work examined the effects of partial closure plus on the mill before installation of oxygen delignification while this work examines the effects of partial closure plus along with oxygen delignification.

Computer simulation shows that partial closure plus reduces AOX, COD, and color in treated bleach plant effluent by 33%, 53%, and 58%, respectively, compared to the mill’s normal operation. Important considerations include treatment of recovery boiler ESP dust to remove chloride and potassium from the recovery cycle, selection of an appropriate addition point for the concentrated alkaline filtrate, and proper management of filtrate flows in the brownstock washing area.