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Using Oxygen and Peroxide to Bleach Kraft Pulps

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Using Oxygen and Peroxide to Bleach Kraft Pulps Using Oxygen and Peroxide to Bleach Kraft Pulps B. VAN LIEROP, N. LIEBERGOTT and M.G. FAUBERT An oxygen-clielation-pero~ide(OQe 15, and removal of transition metal ions from stage to obtain the maximum brightness gain EopQP or QEopP) sequence brightened a the pulp before the peroxide stage. A low from OQP and QEopP sequences. kraft pulp to an IS0 brightness of 70%, kappa pulp is achieved without difficulty in provided that the kappa number of the pulp mills equipped with an oxygen delignifica- entering the peroxide stage was below 17. tion stage. The next chlorination tower and EXPERIMENTAL When the kappa number before P was as low washer can be adapted for a chelation (Q) Various commercially prepared soft- as IO, the brightness afrer QP increased stage [l] or for pulp acidification (A) [6] to wood and hardwood kraft pulps were either linearly to about 79%. High temperatures remove metal ions. The peroxide treatment oxygen-delignified in the mill, or in a spe- (90°C) and long retention times (4 h) in the can be done in the extraction stage or even cially designed laboratoq mixer described P stage increased the brightness of the pulp in the chlorine dioxide towers, provided that elsewhere [8]. The treatments listed in Table provided that the pulp wasfirst chelated and the metallurgy is compatible. This combina- I reduced the kappa numbers by approxi- that residual peroxide remained in the tion of stages is designated OQP, and one mately 30% (Eop) and 45% (0). bleach liquor: The correct combination of version of the process has been named the A chelation treatment with DTPA or H202, NaOH, and DTPA in the "Lignox" process by Eka-Nobel [l]. EDTA was done at 30 min, 50"C, and 3.5 or bleach liquor was necessary to achieve the Those mills lacking an oxygen delig- 2.0% consistency (Table I). The pulp was best brightening response. The QP stages nification stage can combine extended cook- acidified to pH 5 with H2SO4 before adding did not affect the tear-breaking length rela- ing and a peroxide-reinforced low-pressure the chelant to ensure that the pH during the tionship of the handsheets. An enzyme (xyla- oxidative delignification (Eop) [7] to lower chelation was less than 7. The pulp was nase) treatment included as a stage in the the kappa number before bleaching in single washed thoroughly with deionized water OQP or the EopQP sequence improved the or multiple P stages. The chelation stage can after the Q stage. brightness by about I point. be done in the chlorination tower before the The peroxide stage was maintained at Eop delignification stage. The P stages can 10% consistency, 90°C and 240 min when be apportioned between the remaining DED applied to the chelated pulp. These condi- INTRODUCTION towers, with consideration for compatibility tions correspond to those recommended by In order to meet market demands for of materials of construction. This sequence Basta et al. [ 1,4]. Without the chelation stage totally chlorine compound-free (TCF) is designated QEopPP. in the sequence, the conditions were 70°C bleached kraft pulps, bleach plants in some The objective of our work was to and 120 min. The ranges of charges of H202, mills are being configured to use oxygen (0) demonstrate how pulps from various sources NaOH, DTPA, and MgS04 in the peroxide and peroxide (P) stages to brighten pulp respond to chelation and peroxide treatment, bleach liquor used in the experiments are [l-51. These stages can at best produce and to determine the requirements in each listed in Table I. After the P stage, the perox- 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 6.van Lierop, N. LiebergoW, JP and M.G. Faubert pS Paprican 570 St. John's Blvd. Pointe Claire, Que. H9R 3J9 * Now with: Liebergott & Assoc. 4298, 9th St. Laval, Que. H7W 1 Y7 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 7 J193 'I 0 OP OOP A EopOP n. 2'0 QPP 0 OQPP A EopQPP n - u 85 - u 12 A 8. - g 80 z.- g 75 v). v) 70 - E. .F65 - m. 8 10 12 14 16 18 20 22 24 8 10 12 14 16 I8 20 22 24 Kappa before QP No. Kappa No. before QP Fig. 1. The kappa number and brightness obtained after QP and QPP applied to kraft-0 or kraft-Eop pulp delignified to various kappa numbers correlate directly with the kappa number of the pulp entering the QP sequence. Chemical charges and conditions: see Table 1. ide-treated pulp was rinsed with SO2 water gain of 1 1 units. Only 5 brightness units were source of the metal ions in pulp, with secon- at pH 5 during the washing step. gained when a P2 stage was applied to an dary contamination arising from the process OQPI-treated pulp which had a brightness water and equipment [9]. RESULTS AND DISCUSSION of 80%. In chemical pulp bleaching, a chela- Effect of Kappa Number on tion step has been reported to improve the P Stage Brightening Chelation effectiveness of oxygen delignification [ 1I]. Five softwood kraft pulps with kappa A chelation step (Q stage) to remove The acidic pH of a chlorination stage favours numbers ranging from 18.7 to 32.5 were metal ions [9] is commonplace when bright- dissolution of metal ions, so a chelation subjected to OQP, QEopP and/or QP se- ening mechanical pulps with hydrogen per- stage before a peroxide stage is not neces- quences. A second peroxide stage was ap- oxide. Transition metal ions such as iron, sary in a sequence beginning with a chlori- plied to the softwood pulp whose starting manganese and copper greatly accelerate the nation stage. Consequently, when chlorine kappa number was 18.7. This pulp was pro- catalytic decomposition of the active perhy- compounds are eliminated from pulp duced in a pilot plant under extended cook- droxyl ion, presumably through a free-radi- bleaching, the metal ion content in the pulp ing conditions. The remaining pulp samples cal mechanism [IO]. The perhydroxyl ion is becomes an important consideration [12]. were obtained from pulp mills in Canada. considered to be the active component re- The link between manganese content The points in Fig. 1 represent the maximum sponsible for the brightening reaction, and in an oxygen-delignified pulp and peroxide brightness obtained as a result of optimizing any unwanted side reactions which favour bleaching efficiency is shown in Table 11. the conditions in each peroxide stage. The its decomposition waste peroxide and de- Without a chelation stage, the pulp contained H202 charge was 2.5% on pulp (0.d. basis). tract from the brightening process. Metals 35.8 ppm manganese after a failed peroxide The brightness and kappa number occumng naturally in wood are the primary bleaching stage whereas, after successful (Fig. 1) obtained after PI correlated directly with the kappa number of the pulp prior to the QP treatment. The QP stages bleached a pulp at a kappa number of 22 to a brightness of approximately 63%. By lowering the kappa number to IO using a combination of extended cooking and oxygen delignifica- tion, the brightness after QP was as high as 80%. A kappa number of 16 was required to obtain a brightness of about 70% after per- oxide bleaching. The method used to lower the kappa number before the QP stage did not affect the brightening capability of the P stage. The pulp, however, must be chelated before the peroxide bleaching stage, and the proper combination of chemicals and condi- tions must be applied in conjunction with the Hz02 addition in P to obtain maximum brightness gain. The effects of excursions beyond optimum conditions are shown in the subsequent sections. A second P stage applied to a QP-, EopQP- or OQP-treated sample increased the brightness by 5-13 points (open sym- bols, Fig. I), depending on the brightness of the pulp entering the P2 stage. For example, a P2 stage applied to a QP-treated pulp in- creased the brightness from 68 to 79%, a J194 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 7 2% NaOH charge in P(Fig. 2).An excursion beyond optimum alkalinity in P result$ in a ' severe brightness penalty and depletion of peroxide residual. The more heavily chelated pulps (0.6 and 1.2% EDTAon pulp) maintained high brightness levels with ap- plication of 2.5% and even 3.0% NaOH in P. End pH 5.9 6.1 The higher charge of EDTA improved the brightening capability of the P stage by Washing efficiency, Yo =loo') =IO0 10 703) about 1 point with less peroxide consump- tion, and made the pulp tolerant towards a P stage wider range of alkalinity. HzOz consumed, Yo 1.7 1.6 2.3 The effect of pH during the EDTA or the DTPA chelation on the efficiency of the peroxide bleaching was tested on a mill- Q: 0.5% EDTA: 50°C oxygen-delignified softwood kraft pulp P: 2.5% H202,2.5% NaOH; 0.05% MgS04; (kappa no. 14.8) (Table IV). Pulp washing after Q: EDTA and DTPA chelated best when l)well washed the pH of the pulp stock entering the Q stage was below 5 (Table IV). The brightness after P was progressively poorer when the stock pH increased beyond 5. The pH tended to rise slightly during the Q stage as a result of EFFECT OF pH IN THE Q STA the alkalinity of the EDTA or the DTPA solution.
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