Pulp Bleaching Technology

CERNE, 2013

Biotechnology Annual Review, 2006

General concern about the environmental impact of chlorine bleaching effluents has led to a trend towards elementary chlorine-free or totally chlorine free bleaching methods. Considerable interest has been focused on the use of biotechnology in pulp bleaching, as large number of microbes and the enzymes produced by them are known to be capable of preferential degradation of native lignin and complete degradation of wood. Enzymes of the hemicellulolytic type, particularly xylan-attacking enzymes xylanases are now used commercially in the mills for pulp treatment and subsequent incorporation into bleach sequences. Certain white-rot fungi can delignify Kraft pulps increasing their brightness and their responsiveness to brightening with chemicals. The fungal treatments are too slow but the enzymes produced from the fungi can also delignify pulps and these enzymatic processes are likely to be easier to optimize and apply than the fungal treatments. This article presents an overview of the developments in the application of lignin-oxidizing enzymes in bleaching of chemical pulps. The present knowledge of the mechanisms on the action of enzymes as well as the practical results and advantages obtained on the laboratory and industrial scale are discussed.

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.

Enzyme and Microbial Technology, 2004

High-quality flax pulp was bleached in a totally-chlorine-free (TCF) sequence using a laccase-mediator system. Three fungal laccases (from Pycnoporus cinnabarinus, Trametes versicolor and Pleurotus eryngii) and two mediators, 2,2 -azinobis(3-ethylbenzothiazoline-6sulfonic acid) and 1-hydroxybenzotriazole (HBT), were compared. P. cinnabarinus and T. versicolor laccases in the presence of HBT gave the best results in terms of high brightness and low lignin content (kappa number). The former laccase also resulted in the best preservation of cellulose and the largest removal of residual lignin as revealed by analytical pyrolysis, and was selected for subsequent TCF bleaching. Up to 90% delignification and strong brightness increase were attained after a laccase-mediator treatment followed by H 2 O 2 bleaching. This TCF sequence was further improved by applying H 2 O 2 under pressurized O 2 . In this way, we obtained up to 82% ISO brightness (compared with 37% in the initial pulp, and 60% in the peroxide-bleached control) and very low kappa number (near 1). Good results were also found when the laccase-mediator treatment was performed in a bioreactor under pressurized oxygen. The pulp properties obtained, which could not be attained by conventional TCF bleaching of flax pulp, demonstrate the feasibility of enzymatic bleaching to substitute chlorine-containing reagents in manufacturing of these high-price paper pulps.

2015

By removing the primary fines from an oxygen-delignified mill birch pulp, a fiber fraction was obtained having low metals content and no extractives. After DEDeD bleaching the fiber fraction had somewhat higher brightness and better brightness stability than the birch pulp containing the primary fines. The fines fraction was enriched with lignin, extractives, xylan, and metals. Bleaching the fines fraction in a QQP sequence did not affect the extractives, whereas a ZeQP sequence clearly reduced the extractives content. In a biorefinery concept, the fines fraction could be utilized as a source of xylan, fatty acids, sterols, and betulinol. Another possibility is to use the fines fraction unbleached or separately bleached as a bonding material in various fiber furnishes.

Restaurator, 2000

The present study compares five oxidative bleaching treatments (calcium hypochlorite, hydrogen peroxide in two concentrations, light bleaching, and potassium permanganate) and two reductive bleaching treatments (sodium borohydride and tert-butylaminoborane) with each other. Their impact was investigated on pure cellulose (Whatman filter paper) and on two different historic rag papers. The parameters investigated to evaluate the bleaching process are brightness, molecular weight, carbonyl and carboxyl group content. The bleached samples were analyzed directly after the bleaching treatments and additionally after accelerated aging. In summary, all bleaching methods tested lead to a clear increase in brightness, i. e. they are all efficient bleaching agents. Oxidative treatments tend to increase the amount of oxidized cellulose functionalities. However, there is no significant difference between most of the treatments, and all oxidative treatments have caused some damage on cellulose in some occasions. Both reducing treatments successfully decrease the amount of carbonyl groups and partially preserve the cellulose in terms of molecular weight stabilisation. Sodium borohydride is clearly the more efficient reducing agent.

Asian Journal of Chemistry

The effect of each individual bleaching stage on the percentage composition of a-cellulose, acid insoluble lignin and hemicelluloses in multistage total chlorine free bleaching of alkaline sulphate/anthraquinone (AS/AQ) wheat straw pulp was studied by using standard TAPPI and GC methods, respectively. With AOOxP bleaching, > 85 % of pulp fibers were calculated to be composed of true cellulosic fibers, while acid insoluble lignin (%) contents decreased to as low as < 2.0 % as compared to an unbleached sample where pulp fiber constitutes 76.6 % a-cellulose and 7.6 % acid insoluble lignin. A statistical correlation was established and it was found that hemicelluloses are also responsible for ageing of pulp. Burst and tensile index were also found to be effected by changes in hemicelluloses (%) in pulp structure.

Cellulose, 2008

Brewer's spent grain (BSG) was evaluated for bleached pulp production. Two cellulose pulps with different chemical compositions were produced by soda pulping: one from the original raw material and the other from material pretreated by dilute acid. Both of them were bleached by a totally chlorine-free sequence performed in three stages, using 5% hydrogen peroxide in the two initial, and a 0.25 N NaOH solution in the last one. Chemical composition, kappa number, viscosity, brightness and yield of bleached and unbleached pulps were evaluated. The high hemicellulose (28.4% w/w) and extractives (5.8% w/w) contents in original BSG affected the pulping and bleaching processes. However, soda pulping of acid pretreated BSG gave a celluloserich pulp (90.4% w/w) with low hemicellulose and extractives contents (7.9% w/w and \3.4% w/w, respectively), which was easily bleached achieving a kappa number of 11.21, viscosity of 3.12 cp, brightness of 71.3%, cellulose content of 95.7% w/w, and residual lignin of 3.4% w/w. Alkaline and oxidative delignification of acid pretreated BSG was found as an attractive approach for producing high-purity, chlorine-free cellulose pulp.

Journal of wood chemistry …, 2001