Decreasing effluent loads through bleaching modification

TAPPI Journal, 2010

The present investigation meticulously analyzes how oxidative alkaline extraction can be augmented through process changes, and how these augmentations can be leveraged to optimize chlorine dioxide usage with D0(EO)D1, D0(EOP)D1, D0(EO)D1E2D2 and D0(EOP)D1E2D2 sequences for a conventional softwood kraft pulp (24 kappa no.). Bleaching data from Basta and co-workers (1992 TAPPI Pulping Conference) are re-examined and re-interpreted in this study. We determined that ~60 to 65% of the overall ClO2 charge should be applied in the D0-stage. Peroxide addition to an (EOP) can replace 0.6 to 2.5 Kg. ClO2 per Kg H2O2. Boosting the (EO) temperature to 80°C is equivalent to a 70°C (EOP) with 0.25 – 0.30% H2O2, whereas a 90°C (EO) is equivalent to 0.50 – 0.75% H2O2 in a 70°C (EOP). The stoichiometric bleaching data from this study can guide decision-making for lowering chemical usage and minimize costs to reach target brightness levels with three- and five-stage sequences.

Adsorbable organic halogen (AOX) is mainly produced by the reaction of residual lignin and chlo-rinated bleaching agents during bleaching processes. The formation of AOX can be reduced via optimization, addition of chemical additives, and/or pretreatment processes. Only a few studies exist on the effective purification of chlorine dioxide solutions and its effect on AOX reduction. In this paper, the effects of the extraction pressure, temperature, pH, and extraction time on the chlorine gas removal rate (CR) were evaluated. In addition, a response surface model was established to optimize the chlorine gas removal. The temperature of chlorine gas separation was optimized at 39.85 * C, with a separation pressure of 51.20 kPa and solution pH of 1.94, providing an estimated CR of 70.93% under the optimum separation conditions. Chlorine dioxide solutions with and without chlorine gas removal were used for pulp bleaching. AOX formation was reduced by 69.57% at the same 2.0% chlorine dioxide dosage, while the pulp brightness decreased by 3.0%ISO. In addition, AOX formation was reduced by 27.90% at the same bleached pulp brightness (80.00%ISO). The results provide theoretical support for the production of high-purity chlorine dioxide solutions and the development of clean pulping and papermaking processes.

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.

Revista Árvore, 2016

The main objective of this work was to study the technical viability of using the cellulose bleaching effluent, at several stages of the process, seeking fresh water reduction in pulp washing, and evaluating its effect on pulp quality. Eucalyptus spp. industrial cellulosic pulp with oxygen was used in this experiment. The same bleaching sequence D(E+P)DP was performed ten times, under the same conditions (temperature, consistency and time). Counter current washing was used in the bleaching stages, and each sequence was carried out with different washing factors: 9, 6, 3, 0 m3 of distilled water/ton of pulp, trying to reach brightness of 92 ± 0,5% ISO. The ten sequences sought to achieve the stability of the effluent organic load, measured by the chemical oxygen demand (COD). Then, the COD results were compared to the brightness ones from the bleached pulp. The evaluated results from the ten sequences and four different washings showed an increasing in COD due to the organic load acc...

TAPPI Engineering, Pulping & Environmental Conference (Memphis, TN USA; Oct. 11-14, 2009), 2009

Hexenuronic acids (HexA) has been the subject of much research since the mid-1990s when it was discovered that these functional groups can contribute to the measured kappa number of the pulp, as well as to how a pulp respond to various bleaching agents. The goal of this paper is to review and critically analyze the scientific literature concerning HexA. Most quantification methods utilize UV/Vis spectroscopy, in conjunction to selective HexA hydrolysis, to measure the concentration of these structures in pulp. The strengths and weakness of these methods are discussed. HexA contributes approximately 15 to 50% of the measured kappa number for most hardwood pulps, as opposed to ≤ 10% for soft-woods. The formations of these groups, as well as their depletion, during kraft pulping are a strong function of wood species (hardwood vs. softwood) and effective alkali (EA). The activation energies for HexA generation and removal are similar to those related to bulk delignification and hemicellulose degradation. Thus, the manipulation of cooking temperature is difficult to suppress HexA formation while promoting delignification and HexA removal. HexA are not removed in considerable quantities during oxygen delignification or peroxide bleaching stages. These unsaturated structures are degraded when exposed to chlorine dioxide (ClO2). HexA groups contribute to the overall ClO2 consumption during ECF bleaching of hardwood pulps. Experimental evidence from the literature suggests that hypochlorous acid generated from the initial reactions of ClO2 with lignin is the primary agent responsible for HexA removal. Hot A-stages when applied ahead of ECF bleaching can reduce the amount of HexA entering into the bleach plant by 30 to 90%. Part of the kappa number drop observed during the hot A-stage is caused by HexA removal, but some lignin is removed as well. Most applications of hot A-stages are combined with the D0 into a single operation, (A/D0) or "hot D0-Stage." Such processes can be very effective at reducing the total ClO2 needed for ECF bleaching of hardwood pulps (20 to 50%). The cost savings in bleaching with operating a HexA hydrolysis process is a function of the HexA level in the pulp and the additional steam requirements of the hydrolysis stage (i.e., temperature and consistency).

TAPPI Journal, 2013

This investigation evaluates how higher reaction temperatures or oxidant reinforcement of caustic extraction affects chlorine dioxide consumption during elemental chlorine-free (ECF) bleaching of North American hardwood pulps. Bleaching data from Parathasarathy et al. (1993), McDonough et al. (2008) and Reeve and Weishar (1999) were used to develop statistical response surface models for chlorine dioxide delignification and brightening sequence for a variety hardwood pulps. The effects of higher (EO) temperature and of peroxide reinforcement were estimated from observations reported in the scientific literature. The addition of peroxide to an (EO) stage roughly displaces 0.6 to 1.2 kilograms chlorine dioxide per kilogram peroxide used in the ECF bleach sequence. Increasing the (EO) temperature by Δ20°C (e.g., 70 to 90°C) lowers the overall chlorine dioxide demand by 0.4 to 1.5 kilograms. Unlike what is observed for ECF softwood bleaching, the presented findings suggest that hot oxidant-reinforced extraction stages result in somewhat higher bleaching costs when compared to milder alkaline extraction stages for hardwoods. The substitution of an (EOP) in place of (EO) resulted in small changes to the overall bleaching cost. The models employed in this study did not take into account pulp bleaching shrinkage (yield lost) in order to simplify the calculations.

Water Science & Technology, 2007

The hot acid hydrolysis followed by chlorine dioxide (A/D p ) and hot chlorine dioxide (D p ) technologies have proven very useful for bleaching of eucalyptus kraft pulp. Although the characteristics and biodegradability of effluents from conventional chlorine dioxide bleaching are well known, such information is not yet available for effluents derived from hot acid hydrolysis and hot chorine dioxide bleaching. This study discusses the characteristics and biodegradability of such effluents. Combined whole effluents from the complete sequences DEpD, D p EpD, A/D p EpD and ADEpD, and from the pre-bleaching sequences DEp, D p Ep, A/D p Ep and ADEp were characterized by quantifying their colour, AOX and organic load (BOD, COD, TOC). These effluents were also evaluated for their treatability by simulation of an activated sludge system. It was concluded that treatment in the laboratory sequencing batch reactor was efficient for removal of COD, BOD and TOC of all effluents. However, colour increased after biological treatment, with the greatest increase found for the effluent produced using the AD technology. Biological treatment was less efficient at removing AOX of effluents from the sequences with D p , A/D p and AD as the first stages, when compared to the reference D stage; there was evidence of the lower treatability of these organochlorine compounds from these sequences.

October 2013, 2013

This work evaluates the findings of various research efforts and describes the development, use, and benefits of applying techno-economic models of different types to various production and bleach plant scenarios. During the late 1980s and early 1990s, researchers determined that bleach plants were releasing potentially significant quantities of chlorinated organics into receiving waters. This led to extensive research to develop bleaching chemicals to eliminate chlorine and hypochlorite from the bleaching sequence. Over time, 100% chlorine dioxide substitution for chlorine became the industry standard (today’s elemental chlorine free [ECF] bleaching), and research efforts shifted toward chemical optimization of the ECF bleaching sequence. Several delignification stage and brightening stage models were developed and refined for use in bleach plant simulations to optimize chemical performance. Advances in computing power and the availability of packaged spreadsheet and simulation pro...

Tappi Journal

ABSTRACT: 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 ...

Nordic Pulp & Paper Research Journal, 2002

A large number of ECF bleaching sequences were used to bleach an oxygen-delignified Eucalypt kraft pulp to full brightness. An evaluation was made at a reference ISO brightness of 89% and different ways to reduce the use of chlorine dioxide are discussed, including the use of hydrogen peroxide, ozone and treatments in hot acidic medium. It is shown that the use of chlorine dioxide can indeed be reduced, but replacement of other chemicals also affects other parameters such as viscosity and yellowing characteristics. The optimisation of bleaching must also be considered from other perspectives, e.g. yield and different strength properties. Hot treatments utilising chlorine dioxide or only acidic conditions are an efficient way of improving the yellowing characteristics of a pulp, although other pulp properties may suffer from such a treatment.