Optimizing the ECF Bleaching of a High Kappa North American West Coast Softwood Kraft Pulp, 1998 Pulping Conference Proceedings

Washed, softwood brown stock kraft pulp (36.4 Kappa) was laboratory bleached with ECF sequences to optimize bleach chemical cost, fibre strength maintenance and filtrate environmental quality. Do Kappa factors from 0.12 to 0.24 were evaluated. Both extraction stage upflow tube pressure and time at pressure were varied. Percent ClO2 in D1 stage was varied from 0.6 to 1.8 to reach target brightness.

When targeting 85.0% ISO brightness, optimum conditions were: three stages, 0.16 Kappa factor in the Do stage, 0.8% H O in an (EPO) stage operated at 75 psi in the upflow tube and 1.0 %ClO in D1. Lowering Do Kappa 2 2 2 Factor to 0.16 and using a pressurized peroxide stage resulted in reduced bleach chemical cost and AOX, but increased bleach stage filtrate COD by 15 to 20%. After 3 stages of bleaching to an 85 % ISO brightness, fiber strength was independent of Do Kappa factor and extraction stage conditions. Refining energy to reach 400 CSF was reduced when extraction stage peroxide reinforcement was used. Bleach plant AOX decreased 20 to 25% when Do chlorine dioxide was replaced with extraction stage hydrogen peroxide.

At optimum conditions when targeting 85 % ISO brightness, operating costs were reduced 7% relative to a D(EO)D sequence operated at a 0.16 Do Kappa Factor and 35 psia EO upflow tube pressure and 15 minutes retention at pressure. When targeting a fully bleached 90% ISO pulp, optimum conditions were found using a D(EPO)DPD or D(EPO)DPht sequence.