Bleaching Stage Research Articles

The role of bound chlorine in the brightness reversion of bleached hardwood kraft pulp

Our previous paper showed fragmentary evidence that pulp brightness reversion may be negatively affected by its organically bound chlorine (OX) content. A thorough investigation on eucalyptus kraft pulp led to the conclusion that OX increases reversion of certain pulps but this trend is not universal. Alkaline bleaching stages decrease reversion regardless of pulp OX content. Pulps bleached with high temperature chlorine dioxide revert less than those bleached with conventional chlorine dioxide in sequences ending with a chlorine dioxide stage but similarly in sequences ending with a final peroxide stage. The use of secondary condensate for pulp washing decreases reversion.

Performance of a final hydrogen peroxide stage in the ECF bleaching of Eucalypt D0EOPD1 kraft pulps

The compromise between reaching pulp target brightness and guaranteeing a high-quality bleached pulp is often difficult to manage because of the nature of the last chromophores present. This study reports the performance of either hydrogen peroxide or chlorine dioxide in a final bleaching stage applied to three eucalypt D0EOPD1 kraft pulps to reach a target ISO brightness of 90±0.5%. The D1 pulps had ISO brightness values of 84.7%, 86.9%, and 88.7% (pulps A, B, and C, respectively). The experimental results revealed that while chlorine dioxide charge was linearly correlated with D1 brightness, a much higher hydrogen peroxide charge was needed to bleach pulp A compared with pulps B and C. All P pulps had lower brightness reversion and better beatability than D2 pulps. In addition, we observed better physical and optical papermaking properties in P pulps (except for tear index for pulp A), although these pulps exhibited lower intrinsic viscosity. Therefore, the best choice for completing that particular ECF sequence is to push chlorine-dioxide bleaching action in D1 until at least 87% ISO brightness is reached, with the purpose of degrading phenolic units as much as possible, and then to advance with peroxide to boost brightness at the cost of quinone destruction.

Improving chlorine dioxide bleaching efficiency by selecting the optimum pH targets

When operating chlorine dioxide (D) bleaching stages under mill conditions, it is important to establish reasonable operating targets and ranges. We examined the relationship between optimum pH, acceptable range for pH control, bleaching efficiency, and mill specific parameters such as wood species, pulp quality, and chemical charge. The D stages of eight separate fiber lines were simulated in the laboratory using mill pulps obtained from the various bleach plants studied. Both hardwood and softwood species were examined. We examined each simulated D stage over a range of final pH targets and chlorine dioxide charges to determine the relationship of pH to bleaching efficiency. Our work shows that, in practice, the optimum pH for a specific D stage varies with mill specific parameters. We categorized these practical relationships to draw some general conclusions. Mill data from three mill trials were used to validate the relationships.

Peroxomolybdate catalysts in pulp hydrogen peroxide bleaching: Improvement in hexeneuronic acid removal and delignification

We have studied the catalytic effects of peroxomolybdates, i.e., [MoO(O2)2(OH)(H2O)]-, on hydrogen peroxide bleaching on hardwood and softwood pulps in terms of kappa number reduction, hexeneuronic acid (HexA) removal, and delignification. We found that, in the case of hardwood pulp, the decrease of HexA in the peroxomolybdate bleaching stage corresponds to more than 60% of the kappa number reduction obtained in that stage. In the case of pine pulp, inclusion of molybdate in the peroxide stage lowered the kappa number by 4.1. This work also shows that hydrogen peroxide, in the absence of molybdate, did not react to any noticeable extent with neither the lignin model compounds or HexA, under otherwise similar conditions. In order to investigate the oxidation mechanism, lignin model compounds with different side chain structures were reacted with peroxomolybdate. 1H-NMR revealed that lignin model compounds with double bond structure in conjugation with the aromatic ring has an extreme fast speed to react with the peroxomolybdate catalysts systems, presumably through hydroxylation reactions. This mechanism was supported by XPS analysis of the pulp samples, in which the activated hydrogen peroxide was found to act in a similar fashion on surface lignin as well. In conclusion, molybdate activated hydrogen peroxide can be considered a very efficient and selective bleaching agent.

Bagasse alkaline sulfite-anthraquinone (AS/AQ) pulping and totally chlorine free (TCF) bleaching

Abstract Alkaline sulfite/anthraquinone (AS/AQ) pulping and totally chlorine free (TCF) bleaching of bagasse was investigated. Depithed and wet cleaned bagasse was collected from the Pars Paper Company in southern Iran, dried and then pulped. The active alkali charge was chosen between 16 and 20% on oven-dry bagasse and the alkali ratio, Na2SO3/NaOH, was varied from 30/70 to 70/30. The AQ dose was kept constant. While keeping a constant cooking time at maximum temperature for 60 min, the maximum temperature was varied between 155 and 165°C. For bleaching, a pulp with a kappa number of 8.5 was produced under the following conditions: 18% active alkali, alkali ratio 30/70, 60 min cooking at 155°C. TCF bleaching was performed with oxygen and hydrogen peroxide. Oxygen delignification (O) with an alkali charge of 2% NaOH resulted in 50% degree of delignification. A brightness of 80% ISO was achieved with a chelating treatment (Q) and a subsequent oxygen reinforced peroxide bleaching stage (OP) under drastic conditions (4% H2O2, 2.75% NaOH, 90 min at 98°C). With an additional peroxide bleaching stage charging 2% H2O2, the brightness could be further improved to 83.5% ISO. Bleaching led to only small losses in tensile and burst strength at unchanged tear strength. Short beating in a Jokro mill was sufficient to attain good pulp strength. The results indicate that AS/AQ pulping and TCF bleaching can be successfully applied to bagasse to produce pulp for writing and printing paper.

Mathematical model for waste minimization of a bleach plant in paper industry

AbstractIn the present investigation a steady‐state mathematical model is developed for a typical four stage CEHH bleaching sequence in Indian Pulp and Paper Industry. Each stage of the sequence is composed of chemical additions and mixing, reaction in a retention tower, and washing. Unit operation models were formulated using mass balances on liquor, fibres, kappa number, chemicals and COD based on the assumption of perfect mixing and quasi steady‐state. The COD is used in this paper as an indication of the amount of effluent coming out from four bleaching stages. In order to validate the model, the simulated results are compared with the literature data; a good agreement is achieved between the simulated results with the literature data. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pulp bleaching with peracetic acid generated from acetylated polymeric carbohydrates and hydrogen peroxide – Proof of principle

The generation of peracetic acid in the pulp mill may open up new opportunities to use this bleaching chemical. The generation of peracetic acid from acetylated carbohydrates and hydrogen peroxide followed by bleaching of pulp in a T stage was therefore studied with special emphasis on the COD content in the bleaching effluent from this stage. This preliminary work demonstrates that peracetic acid can be generated from acetylated polymeric carbohydrates and hydrogen peroxide, and that this peracetic acid will bleach pulp fibres as effectively as distilled peracetic acid. It is also shown that most of the deacetylated polymeric carbohydrate (in this case cationic starch) adsorbs to the fibre and does not affect the COD content of the filtrate after the T stage. The peracetic acid has been generated with and without pulp present. The peracetic acid yield from polymeric activators and hydrogen peroxide was lower than that from acetylated glucose. The peracetic acid yield can be further be optimised with respect to the acetylated polymer structure, process parameters and added peracetic acid stabilisers. No effect on the tensile strength was observed when cationic starch was used as an acetyl carrier in a TCF sequence with a final (PO) stage. Most of the cationic starch was probably removed in the washing after the (PO) stage pulp. To avoid this, the (PO) stage should be exchanged for a neutral or acidic bleaching stage or the cationic starch should be exchanged to a carbohydrate that better adsorbs to the fibre under alkaline conditions.

Direct method for the determination of phenolic hydroxyl groups in pulp

AbstractFor evaluation of changes in phenolic lignin units during kraft pulping and bleaching, a direct method based on UV reflectance measurements of pulp handsheets was tested. Similar trends in total phenolic hydroxyl-group contents were evident when UV reflectance results for isolated lignins and pulp sheets were compared. This direct method gave reliable results after alkaline cooking, oxygen delignification, and bleaching stages followed by alkaline extraction (DE, ZE). However, the phenolic content obtained after peroxide bleaching (QP) was over-estimated. This may be due to alkali-induced reactions during pH adjustment of the sheets for measurement concomitant with ionisation of the phenolic hydroxyl groups. Otherwise, this direct method based on UV reflectance measurements is a promising new approach for determining the total content of phenolic lignin units in pulp sheets without isolating the lignin.

Comparative effect of ozone, chlorine dioxide, and hydrogen peroxide on lignin: Reactions affecting pulp colour in the final bleaching stage

Abstract Several lignin-like model compounds (vanillin, syringaldehyde, guaiacol, syringol, p-benzoquinone, naphthoquinone) and commercial softwood lignin were submitted to small charges of ozone, chlorine dioxide and hydrogen peroxide in aqueous solution under conditions simulating a final bleaching stage. In the case of ozone, the coloured quinone models were directly destroyed, whereas the phenolic models and lignin underwent a two-step decomposition mechanism: chromophores were formed at very low ozone charges, and then were destroyed with increasing ozone charge. Chlorine dioxide had hardly any effects on the quinone models, but formed coloured groups from the phenolic models and lignin. However, these were more intensely coloured and were only partially removed with higher ClO2 charges. As for hydrogen peroxide, the colour of lignin and naphthoquinone were directly removed, at least partially, but high H2O2 charges were necessary. Consequently, ozone seems to be the best reagent for final bleaching in which small chemical charges are applied. We suggest reaction mechanisms between ozone and the phenolic model compounds and p-benzoquinone that can explain the observations.

Direct Characterization of Hydrogen Peroxide Bleached Thermomechanical Pulp Using Spectroscopic Methods

The effects of thermomechanical pulp (TMP) bleaching with hydrogen peroxide under acidic and alkaline conditions were studied using different spectroscopic analytical methods. The results of hydroxyl radical determination in bleaching solutions, analyses of carbonyl and carboxyl groups contents in the pulp, and the cellulose fiber surface analysis by X-ray photoelectron spectroscopy (XPS) elucidate the chemistry of the hydrogen peroxide treatment. Diffuse reflectance laser flash photolysis (DRLFP) method showed the differences in the photochemical behavior that reflect the changes of the chromophoric system after the preliminary peroxide bleaching stage under acidic conditions. Fourier transform infrared (FTIR) spectroscopy confirmed the non-delignifying character of the bleaching process. Suppression of carbonyl and formation of carboxyl groups in the case of the two-stage peroxide bleaching performed in the presence of catalysts and stabilizers was also confirmed. FT-Raman studies showed the removal of coniferaldehyde groups after treatment under acidic and alkaline conditions.

Fluorescence spectroscopy for chromophore studies on bleached kraft pulps

Abstract Fluorescence techniques are highly sensitive and, since aromatic lignin is the most likely source of natural pulp fluorescence, they are suitable for detailed investigation of residual lignin in bleached pulps. Such investigations are important to our understanding of the bleachability and brightness reversion of pulps. In this study, the effect of bleaching on pulps was evaluated using fluorescence spectra of two softwood kraft pulps bleached in six elemental chlorine-free and totally chlorine-free sequences. Fluorescence spectra were recorded in each bleaching stage using four different instrument settings. In general, pulp fluorescence at excitation wavelengths of 270 and 350 nm increased as bleaching proceeded. Below a certain lignin content, however, pulp fluorescence measured at an excitation wavelength of 430 nm decreased with the residual lignin content. Carbonyl groups have a quenching effect on fluorescence, and the formation of carbonyl groups after oxygen and ozone delignification induced a decrease in pulp fluorescence. The increase in pulp fluorescence after peroxide bleaching stages was due to the removal of carbonyl groups. Reactions of chromophores during brightness reversion also had an effect on pulp fluorescence. These findings demonstrate that fluorescence spectroscopy provides an interesting tool for pulp bleaching studies.

Neural networks in process life cycle profit modelling

Changes in operational environment of the process industry such as decreasing selling prices, increased competition between companies and new legislation, set requirements for performance and effectiveness of the industrial production lines and processes. For the basis of this study, a life cycle profit (LCP) model of a pulp process was constructed using different kind of process information including chemical consumptions and production levels of material and energy flows in unit processes. However, all the information needed in the creation of relevant LCP model was not directly provided by information systems of the plant. In this study, neural networks was used to model pulp bleaching process and fill out missing information and furthermore to create estimators for the alkaline chemical consumption. A data-based modelling approach was applied using an example, where factors affecting the sodium hydroxide consumption in the bleaching stage were solved. The results showed that raw process data can be refined into new valuable information using computational methods and moreover to improve the accuracy of life cycle profit models.

Arundo donax L. reed: New perspectives for pulping and bleaching. 5. Ozone-based TCF bleaching of organosolv pulps

Three selected alkali-based organosolv pulps (alkali-sulfite-anthraquinone-methanol (ASAM), alkali-anthraquinone-methanol (organocell) and ethanol-soda) from agrofibre crop giant reed ( Arundo donax L.) were bleached by an ozone-based TCF (totally chlorine- free) bleaching sequence AZE RQP (where A is an acidic pulp pre-treatment, Z is an ozone stage, (E R) is an alkaline extraction in the presence of reducing agent, Q is a pulp chelating, P is a hydrogen peroxide stage) without oxygen pre-bleaching, and compared with a conventional kraft pulp used as a reference. The different response on bleaching conditions within each bleaching stage was noted for all tested pulps. The pulp bleachability, in terms of brightness improvement or lignin removal per unit of applied chemicals, was found higher for the organocell pulp. The ASAM and ethanol-soda pulps showed the highest bleaching selectivity, expressed by viscosity loss per unit of lignin removed or brightness improved. The overall bleaching results of organosolv pulps were superior to kraft.

Evaluation of white water reuse in the bleaching process for reducing fresh water consumption

The main objective of this work is to study the technical viability of using the effluent generated in paper machines (white water) in the wash presses of the bleaching stage, reducing fresh water consumption. As a case study, the industrial process of Ripasa S.A. Celulose e Papel was evaluated. White water rate is about 700 m3/h and it is not possible to reuse all this volume in the bleaching stage without causing operational problems (fouling in tubes and clogging in the screens). A mass balance of the bleaching unit was developed in an electronic spreadsheet in order to evaluate the possibility of reducing fresh water consumption, using only a fraction of the available white water in the wash presses. To achieve this objective some physical-chemistry properties of the white water stream and of other streams of the process were determined. The maximum concentration of some non-process elements (Si, Ca, Mn and Fe), which could accumulate in the process, were determined in order to establish some parameters to allow process integration of the streams involved, considering operational constraints. The results obtained have shown that it is possible to reduce approximately by 13% the consumption of fresh water and this methodology has been satisfactory.

Treatment of an ECF effluent by combined use of activated sludge and advanced oxidation process

The combined biological and chemical treatments of the cellulose effluents have been studied aiming to promote a more significant degradation of their recalcitrant compounds and to reduce their toxicity, as compared with the isolated treatments. In this work the effluent from acid stages of the ECF bleaching of Eucalyptus urograndis pulp was treated by using separately activated sludge and UV radiation and its combination. The treatment efficiency was evaluated by colour, total phenol, COD, BOD, UV spectroscopy, molar weight distribution and toxicity. The untreated effluent presented 587 +/- 18 CU, 19.3 +/- 0.6 mg.L(-1) of total phenol, 2246 +/- 137 mgO2.L(-1) of COD and 904 +/- 48 mgO2.L(-1) of BOD. It did not show acute toxicity to Escherichia coli, but presented chronic toxicity to Selenastrum capricornutum (EC50 = 25%). The sludge treatment resulted in a colour increasing of 42% and decreasing of total phenol, COD and BOD of 33%, 64% and 92%, respectively. The UV radiation treatment for 120 min resulted in a decrease of colour, total phenol, BOD and COD of 70%, 43%, 62% and 43%, respectively. The combined treatment promoted an expressive decrease for colour and total phenol. The UV absorption indicated a degradation of the aromatic compounds. The biological treatment did not remove chronic toxicity and after UV radiation treatment, a 10 times improving toxicity was noticed.

Differential and synergistic effects of xylanase and laccase mediator system (LMS) in bleaching of soda and waste pulps

Investigation of waste pulps and soda pulp bleaching with xylanase (X) and laccase mediator system (LMS) alone and in conjunction (one after the other) (XLMS). Soda and different grades of waste pulp fibres [used for making three-layered duplex sheets - top layer (TL), protective layer (PL) and bottom layer (BL)] when pretreated with either xylanase (40.0 IU g(-1)) or LMS (up to 200.0 U g(-1)) alone and in combination (one after the other) (XLMS) exhibited an increase in release of reducing sugars [up to 881.0% soda pulp; up to 736.6% (TL), up to 215.7% (PL) and up to 198.0% (BL) waste pulp], reduction in kappa number [up to 17.6% soda pulp; up to 14.0% (TL), up to 25.3% (PL) and up to 10.9% (BL), waste pulp], improvement in brightness [up to 20.4% soda pulp; up to 23.6% (TL), up to 8.6% (PL) and up to 5.0% (BL), waste pulp] when compared with the respective controls. The usage of XLMS along with 15% reduced level of hypochlorite at CEHHXLMS/EHHXLMS bleaching stage reduced kappa number [5.5% soda pulp; 11.4% (TL), 7.9% (PL), waste pulp] and improved brightness [1.0% soda pulp; 0.9% (TL), 1.4% (PL) waste pulp] when compared with the controls. Scanning electron microscopic studies revealed development of cracks, flakes, pores and peeling off the fibres in the enzyme-treated pulp samples. These modifications of the fibre surface during enzymatic bleaching in turn indicated the removal of lignin and derived compounds from the fibre cell wall. The work describes synergistic action of xylanase with LMS for bleaching of waste and nonwood pulps for eco-friendly production of paper and thus reveals a new unexploited arena for enzyme-based pulp bleaching. The drastic improvement in pulp properties obtained after xylanase and LMS treatment would improve the competitiveness of enzyme-based, environmentally benign processes over chemicals both economically and environmentally.

Detoxification of Kraft pulp ECF bleaching effluents by catalytic hydrotreatment

Two different effluents from the D 1 and E 1 stages of the ECF bleaching of Eucalyptus globulus kraft pulp were treated by catalytic hydrogenation in a trickle bed reactor using commercial and homemade Pd/AC catalysts. The reactor was fed with the bleaching effluent and a H 2/N 2 gas stream. The variables studied were space-time (1.4–5 g cat min/mL), gas to liquid flow ratio (286–1000 vol.), gas feed concentration (H 2:N 2, 1:1–1:7.3 vol.), temperature (25–100 °C) and pressure (1–11 bar). Hydrotreatment performance was evaluated in terms of ecotoxicity, adsorbable organic halogen (AOX), chemical oxygen demand (COD), biological oxygen demand (BOD 5) and colour removal. In all the runs, the ecotoxicity of the effluents decreased as a result of the treatment, achieving reductions that ranged from 70% to 98%. Simultaneously to the reduction of toxicity, the hydrotreatment led to a decrease of the colour of the effluents, being the decrease significantly higher in the case of E 1 effluent. The AOX content was reduced by 85% and 23% for E 1 and D 1 effluents, respectively. In the case of D 1 effluent the removal of ecotoxicity was significantly higher than that of AOX, which indicates that much of the toxicity of the effluent must be associated to non-chlorinated organics. In spite of the important reduction of ecotoxicity, the biodegradability of the effluents only increased slightly. The homemade catalysts, prepared from activated carbons with a high external or non-microporous surface area and mesopore volume and a convenient surface chemistry showed a higher efficiency than the commercial one.

Phenol degradation and colour removal in submerged culture of Pleurotus sajor-caju with paper mill effluents

The fungus Pleurotus sajor-caju secretes phenol-oxidases that enable the use of recalcitrant compounds as substrates. The residues of paper manufacture contain high lignin levels, which gives the effluents a characteristic brownish colour. To test the potential of P. sajor-caju cultures on reducing these parameters, we used 90% of raw effluents from medium consistency oxygen delignification and bleaching stages plus 10% of mineral solution and different levels of glucose (5–15 g L−1) as substrate. We observed a greater fungal biomass in cultures using effluent than in controls. Cultures containing 10 to 15 g L−1 of glucose resulted in about 42% colour reduction. The polyphenol content was also reduced by 58.9% by the 13th day of culture. In addition, we observed the secretion of laccases (211.44 U mL−1 and 45.98 U mL−1 using ABTS and syringaldazine, respectively) and peroxidases (6.11 U mL−1-ABTS) both peaking at the 7th day of culture and with similar kinetics of production in different glucose concentrations.

The effect of a catecholate chelator as a redox agent in Fenton-based reactions on degradation of lignin-model substrates and on COD removal from effluent of an ECF kraft pulp mill

We evaluated the effect of a catecholate chelator as a redox agent in Fenton-based reactions (known as chelator-mediated Fenton reaction—CMFR), in the presence of three different transition metals ions (Fe 2+, Fe 3+ and Cu 2+) by determining the oxidative capability of CMFR towards lignin-model substrates. The potential application of mediated Fenton-based reactions as a novel process to treat pulp mill effluent was evaluated and monitored by chemical oxygen demand (COD) and total phenol removals from a combination of the effluents generated during an ECF bleaching stage. The catecholate chelator 3,4-dihydroxiphenilacetic acid (DOPAC) reduced both Fe 3+ and Cu 2+, in addition, the maximum Cu 2+ reduction activity was reached in a shorter time than for Fe 3+ reduction, however, the highest metal reduction activity was observed with Fe 3+. When DOPAC was added to Fenton-based reactions (Fe 3+/H 2O 2, Fe 2+/H 2O 2, Cu 2+/H 2O 2) an increase in oxidative activities of these reactions were found as they resulted in great degradation improvement of the lignin-model substrates azure B, phenol red and syringaldazine. The same increase in oxidative capability of Fenton-based reactions in the presence of DOPAC was observed after effluent treatment, expressed by the increase in COD removal, namely, an increase in the range of about 70% in COD removal when Fe 2+ or Fe 3+ was the catalytic metal and about 25% for Cu 2+. However CMFR lead to an increase in total phenol content. As COD removal by CMFR system using Fe 3+ and Fe 2+ was not significantly different and that Fe 3+ ions promoted lesser increase in total phenol content, Fe 3+ was chosen for experimental optimization. At optimum conditions, 75% of COD and 30% of total phenol removal were achieved.

Impact of xylanase pre-treatment on peroxide bleaching stage of biokraft pulp

Abstract Biokraft pulping of aspen has been investigated. Wood chips were treated with CZ-3 and FP 90031-sp strains of white-rot fungus Ceriporiopsis subvermispora for 2 weeks. Afterwards, bleachability of the pulps in peroxide stage was evaluated. Two commercial xylanase enzymes were used for bleaching. Xylanase pre-treatment of pulps increased their ISO brightness properties by approximately 10%. Biokraft pulps were beaten more easily in comparison to the control kraft pulps. Meanwhile, a significant difference due to the fungal or enzymatic pre-treatments was not found in tensile or bursting strength properties of pulps. A strength correlation was found between freeness and alkaline solubility (S18) for pulps at equal beating conditions.