Optimization of the High Temperature Chlorine Dioxide Stage (DHT) for Hardwood Kraft Pulp, 2001 Pulping Conference Proceedings

K. M. M. Eiras, J. L. Colodette--The impact of DHT stage (D0 at high temperature) conditions (pH, kappa factor, time and temperature) on ECF bleaching of eucalyptus kraft pulp has been evaluated, with the sequence DHTEopD. The most suitable conditions to operate the DHT stage include a 0.22 kappa factor, 3-4 pH, 95C, and 120-180 min. Under these conditions it shows higher delignification efficiency and lower selectivity and brightening action in relation to a conventional D0 stage (30 min, 60 °C) for similar kappa factor and pH values. The DHT stage time and temperature does not significantly affect the optimum pH to run this stage which seems to be in the range of 3-4. The high temperature and time used in the DHT stage decreases metals content and formation of OX in the pulp, and oxalate and AOX in the filtrates. The DHT stage results in improved pulp brightness stability and decreased chlorine dioxide requirement by 10-12%, thus decreasing bleaching costs by about 7%. On the other hand, it decreases overall bleaching yield, viscosity, pulp xylan and HexA´s contents. Furthermore, the DHT stage increases refining energy requirement although it has no significant impact on pulp strength properties.