Optimizing the TCF bleaching of a high Kappa North American west coast softwood kraft pulp

Extensive laboratory evaluations of TCF bleaching on a 28-32 Kappa Douglas fir brownstock kraft pulp were undertaken in order to determine the feasibility of using non-chlorine delignifying and brightening chemicals for the manufacture of market grade bleached pulp at Halsey, Oregon. The pulp was commercially cooked in a M&D digester, washed thoroughly with fresh water and thickened prior to shipment to the testing laboratory. Data collected were analyzed completely to determine response with respect to final pulp brightness, fiber physical characteristics and chemical requirements. Initial work in the studies focused on optimizing oxygen delignification using alkali charge, retention time and hydrogen peroxide addition (between a first and second phase reactor). In this work, an OpO program was identified as providing a maximum amount of Kappa reduction with the minimum loss of fiber strength. Further delignification and brightening on pulp of Kappa 12-14 using one and two stages of ozone preceded and followed by a high-temperature peroxide stage provided pulp of 87-91% ISO. Strength properties of TCF pulp deteriorated as final brightness exceeded about 88% ISO. A comparison to pulp bleached using conventional ECF bleaching, however, showed that TCF pulp strength at 87-87.5% ISO was inferior to ECF pulp by 7-10% when measuring breaking length or burst strength, and 9-11% when measuring tear strength. In addition, based on chemical addition rates, TCF bleaching chemical costs were estimated to be 35-40% higher than ECF costs when comparing pulp of equivalent starting Kappa number and final brightness of 87.0-87.5% ISO.