Modifying the Recycle of Filtrate to Improve the Efficiency of Diffusion Washing in an ECF Bleach Plant, 1999 Pulping Conference Proceedings

Jim Oei & Arvind Thakore

Western Pulp Inc.

Stan Heimburger

Elf Atochem NA

Laboratory studies were done to evaluate rerouting washer filtrate in an ECF bleaching sequence, [D(EO)](EOP)D. The Western Pulp Squamish Operation bleach plant includes four Ahlstrom Kamyr diffusion washers in the first multistage bleach tower being operated in a D(EO) configuration. The second tower is a recently installed Kvaerner pressurized Eop reactor followed by a single stage diffusion washer. The third tower is the D1 Ahlstrom Kamyr tower containing a single stage diffusion washer.

Our work demonstrated that bleaching cost reductions of up to 10% could be achieved by optimizing filtrate recycling management. In addition, a study of simulated lignin precipitation suggests that a modified filtrate recycling scheme could significantly decrease the possibility of diffusion washer screens plugging.

Incorporation of modified filtrate recycle in the bleach plant did proceed upon completion of piping reconfiguration during a planned mill shutdown earlier this year. Preliminary data from about six (6) weeks of operation indicates that bleaching costs have been lowered due to a combination of lower demand for NaOH in the (EOP) stage and reduced use of chlorine dioxide in the sequence. Further optimization is required to realize maximum operating savings.