USING HYDROGEN-PEROXIDE IN 100-PERCENT CHLORINE DIOXIDE BLEACHING SEQUENCES

This report examines the use of hydrogen peroxide in the two E stages of a 100% chlorine dioxide, five-stage D(EO)DED bleaching sequence. The objective was to determine the impact of H2O2 on effluent quality and on bleach-plant chemical costs while maintaining kraft pulp brightness at 90% ISO or higher. Results on softwood pulps showed that EOP or E2P stages could produce 90+% ISO brightness at 0.22 kappa factor, compared with 0.28 kappa factor without H2O2. AOX (adsorbable organic habides) and color in bleach-plant effluent for H2O2-reinforced softwood pulps were about 10% lower than pulps bleached to similar brightness using only ClO2. Chlorinated phenols were 40% lower for H2O2-treated softwood pulps. Hardwood pulps showed similar results, with H2O2-reinforced pulps showing reductions in AOX, color, and phenols of 10%, 15%, and 50%, respectively. For softwood pulps, each kilogram of H2O2 used in an EOP stage reduced ClO2 requirements by 2.3 kg, while each kilogram of H2O2 used in an E2P stage or in high-density storage reduced ClO2 requirements by 5.8 kg. Savings of US$3-5/metric ton could be achieved by using H2O2 to reduce ClO2 requirements. For hardwood, each kilogram of H2O2 used in EOP stage reduced ClO2 requirements by only 0.8 kg, resulting in an increase in bleaching costs of about US$2/metric ton.