Utilization of Kappa Measurement in the D-Stages of ECF-Bleaching, 1996 International Pulp Bleaching Conference Proceedings

Chlorine-free bleaching has made a rapid breakthrough in kraft bleaching, promoted by papermakers and customers demand-ing ECF bleached pulps and paper grades. This change in the process requires development of in-line and on-line bleach-ing measurements and process controls utilizing them. The traditional Dual Sensor control cannot fully meet the new re-quirements: in comparison to chlorine based bleaching, ECF processes involve major changes in process conditions and also the reaction mechanisms and dynamics are different. ECF bleaching has increased the importance of the Do -stage for the entire bleaching result, creating the need for still more accurate measurements. Due to reduced laboratory testing and more reliable automatic on-line Kappa measurements, Kappa-based O-, D0 - and D1 -stage controls have entered modem mills, and strategies integrating Kappa, brightness, and residual meas-urements are becoming a standard in ECF processes. Together with modern control technologies, this enables ever more so-phisticated solutions. This paper presents control results obtained in different bleaching stages and with different measurement combinations. All results come from mill-scale trials made during normal kraft production runs. The benefits and drawbacks of each method, their effects on production economy and product quality are also discussed.