Optimal Bleaching of Non-wood Chemical Pulps with Chlorine Dioxide

摘要

Samples of three unbleached non-wood chemical pulps have been obtained from mills in China and bleached in the laboratory with various elemental chlorine-free (ECF) sequences. All three pulps, but especially bamboo and bagasse, were relatively difficult to bleach compared to hardwood pulps. DED and D(EP)D sequences required high first stage kappa factors to give lowest consumption of chlorine dioxide to 85 or 90 brightness. Using hydrogen peroxide in the extraction stage resulted in considerable saving of chlorine dioxide, up to 10 kg per 5 kg of hydrogen peroxide applied. To achieve 85 brightness with the DED sequence, the optimum kappa factor in the first stage would be at least 0.2 and perhaps higher. To reach 90 brightness with the D(EP)D sequence again the optimum kappa factor would be at least 0.2 and perhaps 0.25. Using oxygen in the extraction stage significantly improved delignification and increased the brightness ceiling. The AOX (absorbable organic halogen) in effluent was somewhat higher than predicted.