Optimal conditions for bleaching Eucalyptus kraft pulp with a three stage sequence

In December 2003 Suzano Papel & Celulose S/A company implemented a three stage sequence to produce fully bleached (88-90% brightness) eucalyptus kraft pulp at the Suzano mill. The experimental procedure described herein aimed at identifying best operational conditions to run the bleach plant, which served as support data for the mill start up. An oxygen delignified eucalyptus kraft pulp of kappa 10.5, viscosity 21.2 mPa.s and brightness 52.3% ISO was used throughout the study. The laboratory study involved the optimization of the most significant variables of the DHT(PO)D sequence, including DHT-stage temperature, kappa factor and pH, (PO)-stage temperature, pH, magnesium sulfate charge and oxygen charge, and D-stage chlorine dioxide charge and pH. It was concluded that optimum DHT stage conditions include a temperature of 80-85 degrees C, kappa factor of 0.25 and pH 3.0. Decreasing pH under 3.0 also results in significant kappa drop but causes pulp darkening and significant viscosity loss. Increasing (PO) stage temperature from 80 to 95 degrees C penalizes the process efficiency and the selectivity; therefore the use of oxygen at this stage is totally unnecessary. The final D-stage of the DHT(PO)D sequence is much more efficient when run at pH 5.5, contrary to the common belief that this stage should be run in the range of pH 3.5-4.0. For optimum conditions as described above, chemical requirements for brightness of 89% ISO were: 9.7 kg/adt ClO2, 4.5 kg/adt H2O2 and 8.6 kg/adt NaOH. The results of the laboratory study have been successfully verified under mill operation for about 12 months. After industrial implementation of the new bleach plant the chlorine dioxide and sodium hydroxide consumptions at the mill decreased by 21, 58% and 18%, respectively.