Using Oxygen and Peroxide to Bleach Kraft Pulps B. VAN LIEROP, N. LIEBERGOTT and M.G. FAUBERT zyx An zyxwvutsrqponm oxygen-clielation-pero~ide (OQe EopQP or QEopP) sequence brightened a kraft pulp to an IS0 brightness zyxwvutsrq of zyxwvutsr 70%, provided that the kappa number of the pulp entering the peroxide stage was below 17. When the kappa number before P was as low as zyxwvutsrq IO, the brightness afrer QP increased linearly to about 79%. High temperatures zyxwvutsr (90°C) and long retention times (4 h) in the P stage increased the brightness of the pulp provided that the pulp wasfirst chelated and that residual peroxide remained in the bleach liquor: The correct combination of H202, NaOH, and DTPA in the bleach liquor was necessary to achieve the best brightening response. The QP stages did not affect the tear-breaking length rela- tionship of the handsheets. An enzyme (xyla- nase) treatment included as a stage in the OQP or the EopQP sequence improved the brightness by about I point. zyxwvutsr INTRODUCTION In order to meet market demands for totally chlorine compound-free (TCF) bleached kraft pulps, bleach plants in some mills are being configured to use oxygen (0) and peroxide (P) stages to brighten pulp [l-51. These stages can at best produce brightnesses of around 80 on softwood kraft pulps and about 85 on hardwood kraft pulps without sacrificing pulp properties. Successful peroxide brightening in a TCF process depends on availability of a pulp with a kappa number preferably below JP zyxwvuts pS zyxwvut 6. van Lierop, zyxwvutsr N. LiebergoW, and M.G. Faubert Paprican 570 St. John's Blvd. Pointe Claire, Que. H9R 3J9 * Now with: Liebergott & Assoc. 4298, 9th St. Laval, Que. H7W 1 Y7 15, and removal of transition metal ions from the pulp before the peroxide stage. A low kappa pulp is achieved without difficulty in mills equipped with an oxygen delignifica- tion stage. The next chlorination tower and washer can be adapted for a chelation (Q) stage [l] or for pulp acidification (A) [6] to remove metal ions. The peroxide treatment can be done in the extraction stage or even in the chlorine dioxide towers, provided that the metallurgy is compatible. This combina- tion of stages is designated OQP, and one version of the process has been named the "Lignox" process by Eka-Nobel [l]. Those mills lacking an oxygen delig- nification stage can combine extended cook- ing and a peroxide-reinforced low-pressure oxidative delignification (Eop) [7] to lower the kappa number before bleaching in single or multiple P stages. The chelation stage can be done in the chlorination tower before the Eop delignification stage. The P stages can be apportioned between the remaining DED towers, with consideration for compatibility of materials of construction. This sequence is designated QEopPP. The objective of our work was to demonstrate how pulps from various sources respond to chelation and peroxide treatment, and to determine the requirements in each stage to obtain the maximum brightness gain from OQP and QEopP sequences. EXPERIMENTAL Various commercially prepared soft- wood and hardwood kraft pulps were either oxygen-delignified in the mill, or in a spe- cially designed laboratoq mixer described elsewhere [8]. The treatments listed in Table I reduced the kappa numbers by approxi- mately 30% (Eop) and 45% (0). A chelation treatment with DTPA or EDTA was done at 30 min, 50"C, and 3.5 or 2.0% consistency (Table I). The pulp was acidified to pH 5 with H2SO4 before adding the chelant to ensure that the pH during the chelation was less than 7. The pulp was washed thoroughly with deionized water after the Q stage. The peroxide stage was maintained at 10% consistency, 90°C and 240 min when applied to the chelated pulp. These condi- tions correspond to those recommended by Basta et al. [ 1,4]. Without the chelation stage in the sequence, the conditions were 70°C and 120 min. The ranges of charges of H202, NaOH, DTPA, and MgS04 in the peroxide bleach liquor used in the experiments are listed in Table I. After the P stage, the perox- JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 7 J193