Bleaching Of Mechanical Pulps Research Articles

Environmental benefits of magnesium hydroxide-based peroxide bleaching of mechanical pulp – mill results

The use of a new, technical, high-purity magnesium hydroxide-based peroxide bleaching additive was evaluated in full mill-scale trial runs on two target brightness levels. Trial runs were conducted at a Finnish paper mill using Norwegian spruce (Picea abies) as the raw material in a conventional pressurized groundwood process, which includes a high-consistency peroxide bleaching stage. On high brightness grades, the use of sodium-based additives cause high environmental load from the peroxide bleaching stage. One proposed solution to this is to replace all or part of the sodium hydroxide with a weaker alkali, such as magnesium hydroxide. The replacement of traditional bleaching additives was carried out stepwise, ranging from 0% to 100%. Sodium silicate was dosed in proportion to sodium hydroxide, but with a minimum dose of 0.5% by weight on dry pulp. The environmental effluent load from bleaching of both low and high brightness pulps was significantly reduced. We observed a 35% to 48% reduction in total organic carbon (TOC), 37% to 40% reduction in chemical oxygen demand (COD), and 34% to 60% reduction in biological oxygen demand (BOD7) in the bleaching effluent. At the same time, the target brightness was attained with all replacement ratios. No interference from transition metal ions in the process was observed. The paper quality and paper machine runnability remained good during the trial. These benefits, in addition to the possibility of increasing production capacity, encourage the implementation of the magnesium hydroxide-based bleaching concept.

IMMOBILIZATION OF LIPASE ON CHITOSAN BEADS FOR REMOVAL OF PITCH PARTICLES FROM WHITEWATER DURING PAPERMAKING

Pitch deposits originating from alkaline peroxide bleaching of mechanical pulps can seriously decrease the runnability of the paper machine when efforts have been made to increase the reuse of process water. In order to degrade pitch particles present in whitewater, lipase was immobilized on chitosan beads using a binary method. The operational stability of the immobilized lipase and its efficacy for treating whitewater were also preliminary studied. The results showed that the highest activity of immobilized lipase was achieved by using 0.5% 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) for activation and 0.0025% glutaraldehyde for cross-linking chitosan. The immobilized lipase also exhibited very good operational stability, and the pitch particles present in whitewater could be reduced by 66.8% after treatment with the immobilized lipase.

Characterization of Secondary and Primary (Hydroxymethyl)Phosphines and Their Oxidation Products: Synergism in Pulp-Bleaching

With the aim of learning about the synergistic effect of a combination of (HOCH2)3P and Na2S2O4 for the bleaching of mechanical pulps, reactions of (HOCH2)3P (and its associated precursor, (HOCH2)4P+Cl−) with Na2S2O4 were studied in aqueous media under Ar and in air. The secondary phosphine (HOCH2)2PH is the major product in a 1:1 reaction of (HOCH2)3P and Na2S2O4 under Ar, the formaldehyde being removed in the form of HOCH2SO3Na and HOCH2SO2Na; the latter is Rongalite, itself an industrial bleaching agent. Significant amounts of the primary phosphine HOCH2PH2 are also seen. When the reaction is performed in air, the oxide (HOCH2)2P(O)H is the main product. Reaction of (HOCH2)4P+Cl− with Na2S2O4 (even under Ar) leads to solely the oxidation products: (HOCH2)3PO, (HOCH2)3PS, and (HOCH2)2P(O)OH. 31P{1H} and 1H NMR spectral data for secondary and primary (hydroxymethyl)phosphines and their oxidation products are reported for the first time. The toxicity of the primary and secondary phosphines is likely to curtail their use in commercial bleaching processes. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.

Stabilization of manganese (Mn)-induced peroxide decomposition

Various ways were explored to reduce Mn-induced peroxide decomposition under conditions similar to industrial bleaching of mechanical pulps. It was found that hydrogen peroxide is more stable if Mn (II) or (III) are stabilized prior to peroxide addition. Sodium silicate/magnesium sulfate solution stabilized hydrogen peroxide in the presence of both Mn(II) and Mn(III), while diethylenetriamine pentaacetic acid (DTPA) effectively stabilized alkaline peroxide in the presence of Mn(II), but not Mn(III).Magnesium hydroxide/sodium silicate solution was not effective in stabilizing both Mn (II) and (III); however, microwave assisted sodium silicate-Mn complex modification resulted in complete peroxide stabilization of both Mn (II) and (III). Key words: Manganese, peroxide decomposition, bleaching, stabilizers, chelant.

Mechanical Pulping: High consistency hydrogen peroxide bleaching of mechanical pulps with varying amounts of fines

Article Mechanical Pulping: High consistency hydrogen peroxide bleaching of mechanical pulps with varying amounts of fines was published on August 1, 2010 in the journal Nordic Pulp & Paper Research Journal (volume 25, issue 3).

Lowering the cationic demand caused by PGA in papermaking by solute adsorption and immobilized pectinase on chitosan beads

Pectins or polygalacturonic acids (PGA) were originated from alkaline peroxide bleaching of mechanical pulps and considered as the dominant troublesome substance, which seriously decreased the runability of the paper machine in process water closure. To find wide application of enzymes in lowering PGA concentration in papermaking industries, cross-linked chitosan beads were prepared. Results showed that the PGA-absorption capability of chitosan beads was greatly affected by its cross-linking degree. The activity of immobilized pectinase on cross-linked chitosan beads were also investigated and the highest activity of binary immobilized pectinase on cross-linked chitosan was achieved using 1.00% of activating reagent or 0.005% of glutaraldehyde. Cationic demand of PGA solutions was obviously lowered by increasing the temperature of enzymatic treatment. GPC analysis showed a sharp decrease in the molecular weight of PGA after enzymatic treatment. Besides, operational stability of the immobilize pectinase was analyzed to evaluate its application potential.

Review: Using magnesium hydroxide as the alkali source for peroxide bleaching of mechanical pulps - process chemistry and industrial implementation

Abstract Magnesium hydroxide, which is a weak alkali, can be used as the alkali source for peroxide bleaching of mechanical pulps. This magnesium hydroxide-based peroxide bleaching process has been commercialized in the paper industry. In this paper, we review the literature results regarding the process chemistry of the Mg(OH)2-based peroxide bleaching process, including the kinetics, peroxide decomposition, anionic trash/COD/oxalate formation, the effect on pulp properties. The benefits associated with the Mg(OH)2- based peroxide bleaching process are discussed. We also discuss the mill implementation of the Mg(OH)2- based peroxide bleaching process in the pulp and paper industry. A case is presented where magnesium hydroxide completely replaces sodium hydroxide as the alkali source. A partial magnesium substitution for sodium hydroxide can also be an option.

Refiner bleaching with magnesium hydroxide (Mg(OH)2) and hydrogen peroxide

Conventional bleaching of mechanical pulp with magnesium hydroxide and hydrogen peroxide shows significant economic benefits over bleaching with caustic soda and sodium silicate. Magnesium hydroxide [Mg(OH)2] bleaching generates lower effluent BOD/COD, higher pulp yield, lower anionic trash in the paper machines, reduced oxalate scaling and, in some cases, higher bulk. NORPAC has developed this bleaching process into a commercial, patented refiner bleaching technology. The excellent mixing, high temperature, and high consistency of the refining environment solubilize the magnesium hydroxide and provide an excellent bleach reactor with the addition of hydrogen peroxide. We have shown that by adding magnesium hydroxide into the refining system, refiner energy demand is reduced by 100-200 KW-hr/odmt and pulp strength is improved by 5-10%. In this paper, we describe NORPAC’s development of refiner bleaching, University of Maine lab refiner studies that support NORPAC’s work, and the additional benefits this technology provides to the mill.

Advanced Effluent Treatment with Ozonation and Biofiltration at the Paper Mill SCA-Graphic Laakirchen AG – Austria: Design and Operation Experience

The paper mill “SCA GRAPHIC LAAKIRCHEN AG” is situated in Austria and produces 486,000 to of graphic paper per year. A yearly amount of 6,923,000 m3 of wastewater with a COD-load of 7,819,000 kg is treated on site (2005). The trend towards brighter paper grades requires more intense bleaching of mechanical pulp and increased usage of chelating agents and optical brighteners. These chemicals are difficult to degrade with biological waste water treatment. Higher bleaching dissolves more lignin, which is hard to degrade as well. As a result the COD removal efficiency in the biological stage of the wastewater treatment plant drops and additional measures for enhanced COD reduction have to be implemented in order to maintain given discharge limits.

Further Understanding on the Cationic Demand of Dissolved Substances During Peroxide Bleaching of a Spruce TMP

Two issues were investigated regarding the cationic demand generated during peroxide bleaching of a spruce TMP. The kinetic study showed that the cationic demand was formed in a rapid process, with the vast majority formed at the very beginning of the reactions. The amount of cationic demand from the conventional peroxide bleaching (the P process) was compared with that from the modified peroxide bleaching process (the PM process). A second aspect of this investigation was on the contribution of oxidized and dissolved lignin to the cationic demand formed. Experimental evidence supports the conclusion that the oxidized and dissolved lignin is an important source of the cationic demand. A methodology was developed to estimate the contribution of each of the dissolved organic components to the total cationic demand formed during peroxide bleaching of mechanical pulp.

Alkaline Darkening and Its Relationship to Peroxide Bleaching of Mechanical Pulp

The effect of alkalinity, transition metals, and oxygen on alkaline darkening of mechanical pulp, and its relations to subsequent peroxide bleaching were further investigated. The chromophores generated under mild conditions of an alkaline treatment can be destroyed in a subsequent peroxide stage. Peroxide‐resistant chromophores are generated only under severe conditions. The results also show that a short alkaline pretreatment can improve the performance of a peroxide bleaching stage.

Relationship between dissolution of fiber materials and development of pulp strength in alkaline peroxide bleaching of mechanical pulp

Abstract This study elucidates the relationship between the dissolution of pulp components and the development of fiber bonding properties in alkaline peroxide bleaching of aspen mechanical pulp. In general, bleaching reactions cause the removal of pulp substances from the fibers, which in turn improves pulp strength properties. Nonetheless, alkaline hydrolysis is particularly important to the development of strength because this reaction mechanism plays a key role in imparting additional carboxylic acid groups onto the fibers. The strong correlation between fiber carboxylic acid group concentration and the amount of anionic dissolved substances makes it possible for us to predict the strength properties of bleached pulps by estimating the anionicity of bleaching filtrates using analytical methods such as cationic demand. The paper also provides insights into the importance of the alkalinity in peroxide bleaching to the dissolution of pulp materials and the strength development.

Formation of Oxalate from the Mg(OH)2‐Based Peroxide Bleaching of Mechanical Pulps

Oxalate is a major source of scaling during the manufacturing process of bleached mechanical pulps and the majority is formed during the peroxide bleaching stage. In this paper, we investigated the effect of using Mg(OH)2 as an alkali source during peroxide bleaching on the formation of oxalate and its partition between soluble and precipitated oxalates. We found that at the same brightness target, the total amount of oxalate formed is similar between the Mg(OH)2‐based peroxide system and the conventional NaOH‐based system, however, almost all of the newly formed oxalate from the former was found in the soluble state, while in the NaOH‐based peroxide process, a large fraction is in the precipitate form. Therefore, the oxalate‐related scaling is significantly less or even negligible during the Mg(OH)2‐based peroxide process. The underlying mechanism accounting for the above observation will be discussed.

Optimization and Control of Hydrogen Peroxide Bleaching

Hydrogen peroxide bleaching is nowadays used not only for mechanical pulp but also for chemical pulp in delignifying OP-or PO-stages (the notification depends of if oxygen or peroxide is the dominating one).The method discussed in this paper is primarily developed for bleaching of mechanical pulp, but is in principal applicable also for bleaching of chemical pulp. If the bleaching is carefully optimized with respect to total alkali, the brightness will be dependent on the peroxide consumption.The peroxide consumption is basically a function of pulp consistency and reaction time during bleaching. Mathematical models developed by Dr. Moldenius for hydrogen peroxide can be used for both one-and two-stage bleach plants.In both cases a system with recycling of residual peroxide is used and compensating the flow of fresh peroxide to obtain a constant feed into the bleach tower.This requires an accurate measurement of the total amount of residual peroxide going back to the input of the bleaching tower. For this purpose, the analyzer RPA-5000 is used. The function of this analyzer will be presented in the beginning of this paper.

Prevention of Fouling to Cation Exchange Membrane in Electrolyzer for On-site Production of Alkaline Hydrogen Peroxide.

Hydrogen peroxide is one of the most important chemicals used in deinking of waste paper and bleaching of mechanical pulp. Environmental restrictions on the use of chlorine-based bleaching techniques are forcing the use of hydrogen peroxide in chemical-pulp bleaching. Therefore a demand for the on-site production of hydrogen peroxide is increasing in the pulp and paper industry. The trickle-bed electrolyzer was developed with a cation-exchange membrane, which could prevent the loss of current efficiency (back-migration of the perhydroxy ion through separator), for the electrochemical production of the hydrogen peroxide. And in order to verify the long-term performance of the electrolyzer, the operational condition of the electrolyzer was investigated. Although the cation-exchange membrane was fouled during continuous operation, we found that an addition of an ethylenediaminetetra-acetic acid (EDTA) salt into the anolyte improved the current efficiency and prevent fouling of the ion-exchange membrane. Anolyte recycling was carried out for a practical usage of the anolyte. The cation-exchange membrane was fouled with metallic compounds from make-up NaOH solution when anolyte was recycled 47 times. These metallic compounds were effectively eliminated from the make-up solution by filtration treatment after mixing with magnesium sulfate. It was also clarified that 10% anolyte purge was effective to keep the lower metal concentration from the critical level. The possibility of the trickle-bed electrolyzer is discussed for the on-site electrochemical production of the hydrogen peroxide.

Microwave enhanced non-alkaline bleaching of mechanical pulps: A new solution to an old problem

Since the granting of our first US patent on microwave induced catalytic decomposition of PCBs some 18 years ago, microwave technology has slowly begun to attract some industrial attention in the last few years. There was no doubt that microwaves had been applied to facilitate the conventional bleaching process of wood pulps in the past but with little or no success. The myth that microwaves can be regarded and manipulated as a rapid heat source probably caused most of the failures of unsuccessful microwave applications. It is therefore necessary to understand the real potential of microwaves as an energy source and to identify the many critical electromagnetic properties of the load (chemical systems), before a final choice of a proper microwave system be designed and installed. In this preliminary report, we demonstrate the real potential of pulsed microwave radiation for non-alkaline bleaching of mechanical pulps. Typically, very high consistency TMP up to 93% can be treated by pulsed microwave in the order of 90 seconds with an average increase of 20 to 25 points in brightness.

Comparing the Environmental Performance of Fluorescent Whitening Agents with Peroxide Bleaching of Mechanical Pulp

SummaryThe use of fluorescent whitening agents (FWAs) instead of oxidative bleaching agents such as peroxide is an alternative for the bleaching of mechanical pulp. By this approach, the chromophores of the wood components in the pulp are not destroyed chemically but the brightness of the pulp is achieved by increased re‐emission of blue light in the range of 400–500 nm. In this study, a typical FWA and peroxide bleaching chemicals are compared with respect to both production and application in the pulp mill. The life‐cycle inventory shows that, on the one hand, the production of the FWA leads to higher releases of salts and adsorbable organically bound halogens (AOX) to surface waters and that, on the other hand, significantly less FWA is required in the application step in order to reach the same pulp brightness. The life‐cycle impact assessment of the production step is presented in terms of Eco‐indicator 95. These results, however, do not cover the environmental fate of various chemicals released to the aquatic environment in the course of the bleaching/whitening step. Therefore, this part is assessed by means of a more detailed investigation of the chemicals' environmental fate in rivers and their aquatic toxicity.

Applications of ESR and CIDEP to mechanistic studies of lignin chemistry

Using time-resolved CIDEP and conventional ESR as well as optical studies much can be elucidated concerning the initial photochemical reactions of lignin model compounds such as α-guaiacoxyacetoveratrone and α-guaiacoxy-β-propioveratrone. The resulting transient radicals are models for intermediates in the processes of photoyellowing of lignin containing mechanical pulps and papers. Model solution experiments which produce evidence for the roles of free radicals in reductive bleaching of mechanical pulps and in the oxidative aging of papers are also described. The more thorough understanding of these complex processes allows the development of strategies to try to “protect” mechanical pulps and papers from light and thermally induced damage and to “repair” any damage which invevitably occurs.

機械パルプと非木材パルプ 機械パルプの光退色とその抑制について

Production of mechanical pulps (MP) have been increased for useful utilization of woods and global environmental problems. However, light-induced color reversion of mechanical pulps is the biggest problem on use of it. But, the problem of the color reversion has not solved yet though many researches on prevention of the color reversion have been conducted.This paper describes about the chemical and biological prevention of light-induced color reversion of mechanical pulps. And furthermore, a mechanism of the color reversion, change of lignins caused by pulping process and effect of the changes on the color reversion, and effect of bleaching of MP on the color reversion are also described.

The Bleaching of Mechanical Pulps with Oxygen and Borohydride

Mechanical pulps are bleached by treatment with oxygen gas and alkali in the presence of borohydrides. The combined treatment gives brightness gains of 15-20 points, comparable with those achieved using stabilized alkaline 4% hydrogen peroxide. By contrast the reaction of oxygen and alkali alone darkens high-yield pulps, and treatment with borohydride in the absence of added oxygen bleaches only partially, to give about half the brightness gained if oxygen is added. Evidence is presented that the bleaching occurs via the conversion of oxygen molecules (O 2 ) to hydroperoxide ions (HOO - ) by reaction of the oxygen with ionized phenolic groups in the lignin. The hydroperoxide ions are thus generated in situ in the lignin where they bleach by reacting with coloured lignin quinone and coniferaldehyde chromophores. Without the addition of borohydride the lignin phenolic groups are irreversibly oxidized by oxygen to coloured products. The primary role of the borohydride is to regenerate the phenolic groups and maintain an oxygen hydroperoxide [(O 2 ) → (HOO - )] cycle. In principle other reducing agents could be used as alternatives but in practice only sodium borohydrides and substituted borohydrides have been found to be sufficiently stable to the reaction conditions. In a secondary role. the borohydride also contributes to the bleaching by reducing radicals, organic peroxides and carbonyl compounds present in the pulps.