INCLUSION OF A PRESSURIZED ACIDOLYSIS STAGE IN CHEMICAL PULP BLEACHING (original) (raw)
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ABSTRACT Laboratory study on bleaching of soda/anthraquinone (SAQ) pulp suggests that AQ catalyzed soda pulp is less responsive towards bleaching chemicals compared to kraft pulp. In this context, shortening of the H-Factor is considered to be one possible strategy to improve bleachability of the SAQ hardwood pulp. To investigate the poorer bleachability, soda, SAQ and kraft pulping were carried out using eight commercially available lignin model compounds representing mostly the reactive species generated during pulping. The investigation indicated that these reactive species vigorously participated in base induced condensation reaction. From GC, GC-MS and solid sample introduction MS analysis, vinylogous compounds were found to undergo Michael addition to repolymerize. Correlating the rate of condensation with alkalinity, some lignin model compounds were found to undergo condensation at a lower rate at 0.6 M NaOH compared to 0.2 M NaOH; which explains the improved final brightness by shortening the pulping time of SAQ/SMAQ pulp. Evidence was also found that the pre extraction (hot water, carbonate or acetate extraction) process removed hydrolysable compounds that could influence pulping and bleaching. Keyword: Anthraquinone, alkaline pulping, lignin condensation, quinone methide, carbanion, Michael addition, preextraction.
Pre-bleaching of mixed hardwood pulp with sulphuric acid
Journal of the Indian Academy of Wood Science, 2010
During kraft pulping, some lignin fragments undergo condensation reactions and form alkali-stable but acid-labile bonds with carbohydrates. Treatment of pulp with an acid can be useful in easy removal of a part of the residual lignin in unbleached kraft pulps. The acid treated pulp can be subsequently bleached by one of the regular bleaching sequences with reduced bleach chemical consumption and environmental pollution loads. In this study, sulphuric acid, with sodium nitrate and sodium nitrite as additives, has been used as a pre-bleaching reagent for mixed-hardwood kraft pulp. The effect of acid treatment on kappa number, CED viscosity, brightness, and strength properties of pulp handsheets has been discussed. The results indicate that sulfuric acid pre-bleaching can be an economical alternative to oxygen alkali pre-bleaching.
THE CHEMISTRY OF BLEACHING AND POST-COLOR FORMATION IN KRAFT PULPS
In this review, the chemical structure of the residual lignin in softwood and hardwood is presented as far as we know it. With this knowledge as a starting point, the basic principles in bleaching of kraft pulp with bleaching agents such as chlorine dioxide, oxygen and hydrogen peroxide is discussed with an emphasis on differences between the behaviour of guaiacyl (softwood) and syringyl-guaiacyl (hardwood) lignin. The importance of other pulp constituents such as hexenuronic acid on the outcome of bleaching operations and various possibilities for removing this structure from the pulp is discussed. In the final part of the presentation, the post-color formation by storage or heat treatment in fully bleached chemical pulp is described and possible means of reducing this nondesirable effect are suggested.
We investigated the bleaching efficiency of the final chlorine dioxide (D2) stage in the D0(EP)D1D2 bleaching sequence, focusing on the effect of pH in relation to the bleaching history of pulp samples. The samples used were unbleached kraft Eucalyptus grandis pulps with kappa no. 14.8 and the same pulp oxygen-delignified to kappa nos. 12 and 9.8. The samples were bleached according to the D0(EP)D1 sequence to a brightness of about 86% ISO and then submitted to the final D2 stage under identical conditions (e.g., chlorine dioxide charge, time, tempera¬ture, and final pH). The target final brightness was 90.5% ISO. Changes in the kappa number, brightness, viscosity, and contents of hexenuronic acid), 4-O-methylglucuronic acid, and the total amount of carboxylic acid (COOH) groups in pulps were monitored during the bleaching sequence. The final brightness of eucalyptus kraft pulps increased when the terminal pH of the D2 stage was raised from 3.0 to 6.5. The 90.5% ISO target brightness in the D2 stage was achieved for all pulps within a pH range of 4.5–6.5, but this required adjusting final pH for individual sam¬ples. The optimal pH value with respect to pulp viscosity was between 3 and 5. Despite dissimilar conditions applied in previous bleaching stages, the samples after the D1 stage revealed similar residual lignin contents as shown by kappa number analysis. The content of hexenuronic acid in the samples, however, varied broadly, from 2 to 26 mmol/kg. Conductometric titration showed different amounts of carboxylic acid groups in pulps after the D1 stage, of which hexenuronic acid accounted for only a minor part. The variations in the fiber charge resulted from the different bleaching conditions applied before the D2 stage. The fiber charge affected the alkali demand in the final D2 stage, whereas variations in the alkali demand affected the initial pH and associated process kinetics. Lower total fiber charge was found to be beneficial for improved final brightening and viscosity when bleached at higher final pH.
Improving the Bleaching Process of Hardwood Chemi-Mechanical Pulp
2016
In the first phase, the effect of different NaOH to H 2 O 2 ratios on bleaching chemi-mechanical pulp (CMP) was investigated. The pH of unbleached pulp was 5.25 and its brightness and freeness were 37.5 and 745 ml respectively. Results indicated that the brightness increased by growing the peroxide charge, but the increase in the NaOH percentage had inhomogeneous effects. As data showed, the best brightness values were found with 0.75 NaOH/H 2 O 2 ratios. High ratios led to a noticeable upward trend in pulp yellowness, while ratios below 0.75 resulted in lower brightness. In the second phase, the effects of replacing NaOH by Mg(OH) 2 were examined. Results showed that this replacement determined an improvement in the optical properties and pulp yield. However, the mechanical properties were similar in both processes. Also, this replacement led to a decrease in the Chemical Oxygen Demand (COD) and conductivity in the bleaching effluent. The residual peroxide in the Mg(OH) 2 bleaching process was higher than in the NaOH bleaching process.
CHARACTERIZATION OF EFFLUENTS FROM PEROXIDE AND PERACETIC ACID BLEACHING OF SOFTWOOD KRAFT PULP
The organic material dissolved from oxygen-alkali delignified softwood kraft pulp in the bleach liquors during the molybdate-activated hydrogen peroxide (mP) and peracetic acid (PAA) stages separately and combined with the subsequent alkaline hydrogen peroxide stage (P) was characterized. In each case, carbohydrate-derived monosaccharides obtained by acid hydrolysis were analyzed by gas chromatography/mass spectrometry (GC/MS) and the molecular-mass distribution of lignin-derived material was determined by gel permeation chromatography (GPC). In addition, the dissolution of carbohydrates and the formation of aliphatic carboxylic acids (determined by GC/MS) were monitored during the mP-P bleaching sequence. The results indicated that a brightness level of 82-85 % ISO could be easily achieved by different bleaching sequences (PAA-P, mP-P, and mP/PAA-P), and that changes in the bleaching agents resulted in many features characteristic of the chemical composition of bleaching effluents.
Influence of Beech Wood Pre-Extraction on Bleaching and Strength Properties of Kraft Pulps
2016
Kraft pulps prepared from beech chips, with 10% wood weight loss, pre-extracted with green liquor solution, hot water and dilute oxalic acid, were more effectively delignified by a double-stage oxygen system in comparison with the reference pulp from original chips. The highest delignification efficiency was demonstrated for pulps prepared from dilute oxalic acid and hot water pre-extracted chips. The bleachability of oxygen delignified kraft pulps in the sequence D0(EO)D1D2 was better in the case of the pulps from pre-extracted chips, first for those extracted with dilute oxalic acid and then for the hot water extracted ones. The consumption of ClO2 in bleaching was lower for these pulps by about 40% and 20%, respectively, while for the pulp from green liquor solution pre-extracted chips it was lower only by about 5% compared with the reference pulp. Brightness reversion of the bleached pulps from pre-extracted chips was lower in comparison with the reference pulp, which correlates...
BLEACHABILITY OF KRAFT PULPS TREATED BY ETHANOL-BASED CHLORINE DIOXIDE DELIGNIFICATION SEQUENCES
Journal of Wood Chemistry and Technology, 2001
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