TCF Bleaching of Eucalyptus Kraft Pulp: The Selection of the Right Sequence and the Best Conditions, 1996 International Pulp Bleaching Conference Proceedings

TCF bleaching was conducted with conventionally oxygen delignified pulp after continuous pulping. The effectiveness of the activation of a second oxygen delignification stage with ozone or peracid was tested. The residual lignin amount was lowered either with an O-AQ -Z- O or an O-AQ -Peracid-O treatment. Residual lignin levels between Kappa number 3 and 5 were achieved without any viscosity penalty. The bleachability was very good after an ozone activation. The application of 0,25% ozone had a better effect compared with a treatment with 1,5% peracetic acid. Final brightness was above 90% ISO at similar strength properties. Residual lignin levels between kappa 3 to kappa 5 after the second oxygen treatment permit final bleaching with hydrogen peroxide in only one stage. An additional acid wash treatment before the peroxide stage decreased the required amount of H2 02 . The bleaching results of the peroxide stage were improved with the addition of small amounts of mag-nesium salts and sodium silicate. The lowest hydrogen per-oxide demand resulted at bleaching temperatures below 100°C with a retention time up to 24 hours. The 90% ISO brightness level is achieved with an input of less than 2% H2 O2 . Bleaching at 110°C accelerates the reaction to completion within 2 hours, but yields inferior results in terms of peroxide consumption and viscosity. 3% H2 O2 are required to produce 89% ISO brightness.