Bleaching of wood pulp

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Bleaching of wood pulp is the chemical processing of wood pulp to lighten its color and whiten the pulp. The primary product of wood pulp is paper, for which whiteness (similar to, but distinct from brightness) is an important characteristic. [1] These processes and chemistry are also applicable to the bleaching of non-wood pulps, such as those made from bamboo or kenaf.

Contents

Paper brightness

Brightness is the amount of incident light reflected from paper under specified conditions, [2] usually reported as the percentage of light reflected, so a higher number means a brighter or whiter paper. In the US, the TAPPI T 452 [3] or T 525 standards are used. The international community uses ISO standards.

Comparison of the US and ISO systems rating high-brightness papers
TAPPI
brightness		ISO
brightness
8488
92104
96108
97≥109

The table shows how the two systems rate high-brightness papers, but there is no simple way to convert between the two systems because the test methods are so different. [4] The ISO rating is higher and can be over 100. This is because contemporary white paper incorporates fluorescent whitening agents (FWA). Because the ISO standard only measures a narrow range of blue light, it is not directly comparable to human vision of whiteness or brightness. [5]

Newsprint ranges from 55 to 75 ISO brightness. [6] Writing and printer paper would typically be as bright as 104 ISO.

While the results are the same, the processes and fundamental chemistry involved in bleaching chemical pulps (like kraft or sulfite) are very different from those involved in bleaching mechanical pulps (like stoneground, thermomechanical or chemo-thermomechanical). Chemical pulps contain very little lignin, while mechanical pulps contain most of the lignin that was present in the wood used to make the pulp. Lignin is the main source of color in pulp due to the presence of a variety of chromophores naturally present in the wood or created in the pulp mill.

Bleaching mechanical pulps

Mechanical pulp retains most of the lignin present in the wood used to make the pulp and thus contain almost as much lignin as they do cellulose and hemicellulose. It would be impractical to remove this much lignin by bleaching, and undesirable since one of the big advantages of mechanical pulp is the high yield of pulp based on wood used. Therefore, the objective of bleaching mechanical pulp (also referred to as brightening) is to remove only the chromophores (color-causing groups). This is possible because the structures responsible for color are also more susceptible to oxidation or reduction.

Alkaline hydrogen peroxide is the most commonly used bleaching agent for mechanical pulp. The amount of base such as sodium hydroxide is less than that used in bleaching chemical pulps and the temperatures are lower. These conditions allow alkaline peroxide to selectively oxidize non-aromatic conjugated groups responsible for absorbing visible light. The decomposition of hydrogen peroxide is catalyzed by transition metals, and iron, manganese and copper are of particular importance in pulp bleaching. The use of chelating agents like EDTA to remove some of these metal ions from the pulp prior to adding peroxide allows the peroxide to be used more efficiently. Magnesium salts and sodium silicate are also added to improve bleaching with alkaline peroxide. [7]

Sodium dithionite (Na2S2O4), also known as sodium hydrosulfite, is the other main reagent used to brighten mechanical pulps. In contrast to hydrogen peroxide, which oxidizes the chromophores, dithionite reduces these color-causing groups. Dithionite reacts with oxygen, so efficient use of dithionite requires that oxygen exposure be minimized during its use. [2]

Chelating agents can contribute to brightness gain by sequestering iron ions, for example, as EDTA complexes, which are less colored than the complexes formed between iron and lignin. [2]

The brightness gains achieved in bleaching mechanical pulps are temporary, since almost all of the lignin present in the wood is still present in the pulp. Exposure to air and light can produce new chromophores from this residual lignin. [8] This is why newspaper yellows as it ages. Yellowing also occurs due to the acidic sizing.[ clarification needed ]

Bleaching of recycled pulp

Hydrogen peroxide and sodium dithionite are used to increase the brightness of deinked pulp. [9] The bleaching methods are similar for mechanical pulp, in which the goal is to make the fibers brighter.

Bleaching chemical pulps

Chemical pulps, such as those from the kraft process or sulfite pulping, contain much less lignin than mechanical pulps, (<5% compared to approximately 40%). The goal in bleaching chemical pulps is to remove essentially all of the residual lignin, hence the process is often referred to as delignification. Sodium hypochlorite (household bleach) was initially used to bleach chemical pulps, but was largely replaced in the 1930s by chlorine. Concerns about the release of organochlorine compounds into the environment prompted the development of elemental chlorine free (ECF) and totally chlorine free (TCF) bleaching processes.

Delignification of chemical pulps is frequently composed of four or more discrete steps, with each step designated by a letter: [10]

Delignification steps of chemical pulps
Chemical or process usedLetter designation
Chlorine C
Sodium hypochlorite H
Chlorine dioxide D
Extraction with sodium hydroxide E
Oxygen O
Alkaline hydrogen peroxide P
Ozone Z
Chelation to remove metalsQ
Enzymes (especially xylanase)X
Peracids (peroxy acids)Paa
Sodium dithionite (sodium hydrosulfite)Y

A bleaching sequence from the 1950s could look like CEHEH – the pulp would have been exposed to chlorine, extracted (washed) with a sodium hydroxide solution to remove lignin fragmented by the chlorination, treated with sodium hypochlorite, washed with sodium hydroxide again and given a final treatment with hypochlorite. An example of a modern totally chlorine-free (TCF) sequence is OZEPY, where the pulp would be treated with oxygen, then ozone, washed with sodium hydroxide, then treated in sequence with alkaline peroxide and sodium dithionite.

Chlorine and hypochlorite

Chlorine replaces hydrogen on the aromatic rings of lignin via aromatic substitution, oxidizes pendant groups to carboxylic acids and adds across carbon carbon double bonds in the lignin sidechains. Chlorine also attacks cellulose, but this reaction occurs predominantly at pH = 7, where un-ionized hypochlorous acid, HClO, is the main chlorine species in solution. [11] To avoid excessive cellulose degradation, chlorination is carried out at pH < 1.5.

Cl2 + H2O ⇌ H+ + Cl− + HClO

At pH > 8 the dominant species is hypochlorite, ClO−, which is also useful for lignin removal. Sodium hypochlorite can be purchased or generated in situ by reacting chlorine with sodium hydroxide:

2 NaOH + Cl2 ⇌ NaOCl + NaCl + H2O

The main objection to the use of chlorine for bleaching pulp is the large amounts of soluble organochlorine compounds produced and released into the environment.

Chlorine dioxide

Chlorine dioxide, ClO2 is an unstable gas with moderate solubility in water. It is usually generated in an aqueous solution and used immediately because it decomposes and is explosive in higher concentrations. It is produced by reacting sodium chlorate with a reducing agent like sulfur dioxide:

2 NaClO3 + H2SO4 + SO2 → 2 ClO2 + 2 NaHSO4

Chlorine dioxide is sometimes used in combination with chlorine, but it is used alone in ECF (elemental-chlorine-free) bleaching sequences. It is used at moderately acidic pH (3.5 to 6). The use of chlorine dioxide minimizes the amount of organochlorine compounds produced. [8] Chlorine dioxide (ECF technology) currently is the most important bleaching method worldwide. About 95% of all bleached kraft pulp is made using chlorine dioxide in ECF bleaching sequences. [12]

Extraction or washing

All bleaching agents used to delignify chemical pulp, with the exception of sodium dithionite, break lignin down into smaller, oxygen-containing molecules. These breakdown products are generally soluble in water, especially if the pH is greater than 7 (many of the products are carboxylic acids). These materials must be removed between bleaching stages to avoid excessive use of bleaching chemicals, since many of these smaller molecules are still susceptible to oxidation. The need to minimize water use in modern pulp mills has driven the development of equipment and techniques for the efficient use of available water. [13]

Oxygen

Oxygen exists as a ground-state triplet, which is relatively unreactive and needs free radicals or very electron-rich substrates such as deprotonated lignin phenolic groups. The production of these phenoxide groups requires that delignification with oxygen be carried out under very basic conditions (pH > 12). The reactions involved are primarily single-electron (radical) reactions. [14] [15] Oxygen opens rings and cleaves sidechains, giving a complex mixture of small oxygenated molecules. Transition-metal compounds, particularly those of iron, manganese and copper, which have multiple oxidation states, facilitate many radical reactions and impact oxygen delignification. [16] [17] While the radical reactions are largely responsible for delignification, they are detrimental to cellulose. Oxygen-based radicals, especially hydroxyl radicals, HO•, can oxidize hydroxyl groups in the cellulose chains to ketones, and under the strongly basic conditions used in oxygen delignification, these compounds undergo reverse aldol reactions, leading to cleavage of cellulose chains. Magnesium salts are added to oxygen delignification to help preserve the cellulose chains, [16] but mechanism of this protection has not been confirmed.

Hydrogen peroxide

Using hydrogen peroxide to delignify chemical pulp requires more vigorous conditions than for brightening mechanical pulp. Both pH and temperature are higher when treating chemical pulp. The chemistry is very similar to that involved in oxygen delignification, in terms of the radical species involved and the products produced. [18] Hydrogen peroxide is sometimes used with oxygen in the same bleaching stage, and this give the letter designation Op in bleaching sequences. Redox-active metal ions, particularly manganese, Mn(II/IV), catalyze the decomposition of hydrogen peroxide, so some improvement in the efficiency of peroxide bleaching can be achieved if the metal levels are controlled. [19]

Ozone

Ozone is a very powerful oxidizing agent, and the biggest challenge in using it to bleach wood pulp is to get sufficient selectivity so that the desirable cellulose is not degraded. Ozone reacts with the carbon–carbon double bonds in lignin, including those within aromatic rings. In the 1990s ozone was touted as good reagent to allow pulp to be bleached without any chlorine-containing chemicals (totally chlorine-free, TCF). The emphasis has changed, and ozone is seen as an adjunct to chlorine dioxide in bleaching sequences not using any elemental chlorine (elemental-chlorine-free, ECF). Over 25 pulp mills worldwide have installed equipment to generate and use ozone. [20]

Chelant wash

The effect of transition metals such as Mn on some of the bleaching stages has already been mentioned. Sometimes it is beneficial to remove some of these redox-active metal ions from the pulp by washing the pulp with a chelating agent such as EDTA or DTPA. [21] This is more common in TCF bleaching sequences for two reasons: the acidic chlorine or chlorine dioxide stages tend to remove metal ions (metal ions usually being more soluble at lower pH), and TCF stages rely more heavily on oxygen-based bleaching agents, which are more susceptible to the detrimental effects of these metal ions. Chelant washes are usually carried out at or near pH = 7. Lower-pH solutions are more effective at removing redox-active transition metals (Mn, Fe, Cu), but also remove most of the beneficial metal ions, especially magnesium. [22] A negative impact of chelating agents, as DTPA, is their toxicity for the activated sludges in the treatment of kraft pulping effluent. [23]

Other bleaching agents

A variety of less common bleaching agents have been used on chemical pulps. They include peroxyacetic acid, [24] peroxyformic acid, [24] potassium peroxymonosulfate (oxone), [24] dimethyldioxirane, [25] which is generated in situ from acetone and potassium peroxymonosulfate, and peroxymonophosphoric acid. [26]

Enzymes like xylanase have been used in pulp bleaching [24] to increase the efficiency of other bleaching chemicals. It is believed that xylanase does this by cleaving lignin–xylan bonds to make lignin more accessible to other reagents. [2] It is possible that other enzymes such as those used by fungi to degrade lignin may be useful in pulp bleaching. [27]

Environmental considerations

The bleaching of chemical pulps has the potential to cause significant environmental damage, primarily through the release of organic materials into waterways. Pulp mills are almost always located near large bodies of water because they require substantial quantities of water for their processes. An increased public awareness of environmental issues from the 1970s and 1980s, as evidenced by the formation of organizations like Greenpeace, influenced the pulping industry and governments to address the release of these materials into the environment. [28]

Worldwide pulp production by type of bleaching used: chlorine (Cl2), elemental chlorine free (ECF) and total chlorine free (TCF) World pulp bleaching 1990-2005.svg
Worldwide pulp production by type of bleaching used: chlorine (Cl2), elemental chlorine free (ECF) and total chlorine free (TCF)

Conventional bleaching using elemental chlorine produces and releases into the environment large amounts of chlorinated organic compounds, including chlorinated dioxins. [29] Dioxins are recognized as a persistent environmental pollutant, regulated internationally by the Stockholm Convention on Persistent Organic Pollutants.

Dioxins are highly toxic, and health effects on humans include reproductive, developmental, immune and hormonal problems. They are known to be carcinogenic. Over 90% of human exposure is through food, primarily meat, dairy, fish and shellfish, as dioxins accumulate in the food chain in the fatty tissue of animals. [30]

As a result, from the 1990s onwards, the use of elemental chlorine in the delignification process was substantially reduced and replaced with ECF (elemental chlorine free) and TCF (totally chlorine free) bleaching processes. In 2005, elemental chlorine was used in 19–20% of kraft pulp production globally, down from over 90% in 1990. 75% of kraft pulp used ECF, with the remaining 5–6% using TCF. [31] Most TCF pulp is produced in Sweden and Finland for sale in Germany, [31] all markets with a high level of environmental awareness. In 1999, TCF pulp represented 25% of the European market. [32]

TCF bleaching, by removing chlorine from the process, reduces chlorinated organic compounds to background levels in pulp-mill effluent. [33] ECF bleaching can substantially reduce but not fully eliminate chlorinated organic compounds, including dioxins, from effluent. While modern ECF plants can achieve chlorinated organic compounds (AOX) emissions of less than 0.05 kg per tonne of pulp produced, most do not achieve this level of emissions. Within the EU, the average chlorinated organic compound emissions for ECF plants is 0.15 kg per tonne. [34]

However, there has been disagreement about the comparative environmental effects of ECF and TCF bleaching. Some researchers found that there is no environmental difference between ECF and TCF, [35] while others concluded that among ECF and TCF effluents before and after secondary treatment, TCF effluents are the least toxic. [36]

See also

Related Research Articles

<span class="mw-page-title-main">Chlorine</span> Chemical element with atomic number 17 (Cl)

Chlorine is a chemical element; it has symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is a yellow-green gas at room temperature. It is an extremely reactive element and a strong oxidising agent: among the elements, it has the highest electron affinity and the third-highest electronegativity on the revised Pauling scale, behind only oxygen and fluorine.

<span class="mw-page-title-main">Pulp (paper)</span> Fibrous material used notably in papermaking

Pulp is a fibrous lignocellulosic material prepared by chemically, semi-chemically or mechanically producing cellulosic fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemicals or plant-based additives, pulp is the major raw material used in papermaking and the industrial production of other paper products.

<span class="mw-page-title-main">Paperboard</span> Thick paper-based material

Paperboard is a thick paper-based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker than paper and has certain superior attributes such as foldability and rigidity. According to ISO standards, paperboard is a paper with a grammage above 250 g/m2, but there are exceptions. Paperboard can be single- or multi-ply.

<span class="mw-page-title-main">Chlorine dioxide</span> Chemical compound

Chlorine dioxide is a chemical compound with the formula ClO2 that exists as yellowish-green gas above 11 °C, a reddish-brown liquid between 11 °C and −59 °C, and as bright orange crystals below −59 °C. It is usually handled as an aqueous solution. It is commonly used as a bleach. More recent developments have extended its applications in food processing and as a disinfectant.

<span class="mw-page-title-main">Kraft process</span> Process of converting wood into wood pulp

The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibres, the main component of paper. The kraft process involves treatment of wood chips with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na2S), known as white liquor, that breaks the bonds that link lignin, hemicellulose, and cellulose. The technology entails several steps, both mechanical and chemical. It is the dominant method for producing paper. In some situations, the process has been controversial because kraft plants can release odorous products and in some situations produce substantial liquid wastes.

<span class="mw-page-title-main">Sodium chlorate</span> Chemical compound

Sodium chlorate is an inorganic compound with the chemical formula NaClO3. It is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 300 °C to release oxygen and leaves sodium chloride. Several hundred million tons are produced annually, mainly for applications in bleaching pulp to produce high brightness paper.

<span class="mw-page-title-main">Pulp mill</span> Facility which pulps wood or plant fibre

A pulp mill is a manufacturing facility that converts wood chips or other plant fiber sources into a thick fiber board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical, or fully chemical methods. The finished product may be either bleached or non-bleached, depending on the customer requirements.

<span class="mw-page-title-main">Kraft paper</span> Paper or paperboard produced from chemical pulp produced in the kraft process

Kraft paper or kraft is paper or paperboard (cardboard) produced from chemical pulp produced in the kraft process.

<span class="mw-page-title-main">Pentetic acid</span> DTPA: aminopolycarboxylic acid

Pentetic acid or diethylenetriaminepentaacetic acid (DTPA) is an aminopolycarboxylic acid consisting of a diethylenetriamine backbone with five carboxymethyl groups. The molecule can be viewed as an expanded version of EDTA and is used similarly. It is a white solid with limited solubility in water.

An antichlor is a substance used to decompose residual hypochlorite or chlorine after chlorine-based bleaching, in order to prevent ongoing reactions with, and therefore damage to, the material that has been bleached. Antichlors include sodium bisulfite, potassium bisulfite, sodium metabisulfite, sodium thiosulfate, and hydrogen peroxide.

<span class="mw-page-title-main">Bleach</span> Chemicals used to whiten or disinfect

Bleach is the generic name for any chemical product that is used industrially or domestically to remove colour (whitening) from fabric or fiber or to disinfect after cleaning. It often refers specifically to a dilute solution of sodium hypochlorite, also called "liquid bleach".

The sulfite process produces wood pulp that is almost pure cellulose fibers by treating wood chips with solutions of sulfite and bisulfite ions. These chemicals cleave the bonds between the cellulose and lignin components of the lignocellulose. A variety of sulfite/bisulfite salts are used, including sodium (Na+), calcium (Ca2+), potassium (K+), magnesium (Mg2+), and ammonium (NH4+). The lignin is converted to lignosulfonates, which are soluble and can be separated from the cellulose fibers. For the production of cellulose, the sulfite process competes with the Kraft process which produces stronger fibers and is less environmentally costly.

<span class="mw-page-title-main">Paper</span> Material for writing, printing, etc.

Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses, or other vegetable sources in water, draining the water through a fine mesh leaving the fibre evenly distributed on the surface, followed by pressing and drying. Although paper was originally made in single sheets by hand, almost all is now made on large machines—some making reels 10 metres wide, running at 2,000 metres per minute and up to 600,000 tonnes a year. It is a versatile material with many uses, including printing, painting, graphics, signage, design, packaging, decorating, writing, and cleaning. It may also be used as filter paper, wallpaper, book endpaper, conservation paper, laminated worktops, toilet tissue, currency, and security paper, or in a number of industrial and construction processes.

<span class="mw-page-title-main">Environmental impact of paper</span>

The environmental impact of paper are significant, which has led to changes in industry and behaviour at both business and personal levels. With the use of modern technology such as the printing press and the highly mechanized harvesting of wood, disposable paper became a relatively cheap commodity, which led to a high level of consumption and waste. The rise in global environmental issues such as air and water pollution, climate change, overflowing landfills and clearcutting have all lead to increased government regulations. There is now a trend towards sustainability in the pulp and paper industry as it moves to reduce clear cutting, water use, greenhouse gas emissions, fossil fuel consumption and clean up its influence on local water supplies and air pollution.

Elemental chlorine free (ECF) is a technique that uses chlorine dioxide for the bleaching of wood pulp. It does not use elemental chlorine gas during the bleaching process and prevents the formation of dioxins and dioxin-like compounds, carcinogens. The traditional ECF sequence is DEopDEpD using the common letter symbols for bleaching stages, though many improved sequences are available.

In industrial paper-making processes, organosolv is a pulping technique that uses an organic solvent to solubilise lignin and hemicellulose. It has been considered in the context of both pulp and paper manufacture and biorefining for subsequent conversion of cellulose to fuel ethanol. The process was invented by Theodor Kleinert in 1968 as an environmentally benign alternative to kraft pulping.

<span class="mw-page-title-main">Paper chemicals</span> Chemicals used in paper manufacturing

Paper chemicals designate a group of chemicals that are used for paper manufacturing, or modify the properties of paper. These chemicals can be used to alter the paper in many ways, including changing its color and brightness, or by increasing its strength and resistance to water. The chemicals can be defined on basis of their usage in the process.

<span class="mw-page-title-main">Chlorine-releasing compounds</span>

Chlorine-releasing compounds, also known as chlorine base compounds, is jargon to describe certain chlorine-containing substances that are used as disinfectants and bleaches. They include the following chemicals: sodium hypochlorite, chloramine, halazone, and sodium dichloroisocyanurate. They are widely used to disinfect water and medical equipment, and surface areas as well as bleaching materials such as cloth. The presence of organic matter can make them less effective as disinfectants. They come as a liquid solution, or as a powder that is mixed with water before use.

A peroxide-based bleach or simply peroxide bleach is any bleach product that is based on the peroxide chemical group, namely two oxygen atoms connected by a single bond, (–O–O–). This bond is fairly weak and is often broken in chemical reactions of peroxides, giving rise to very reactive oxygen species, which are the active agents of the bleach.

Mechanical pulping is the process in which wood is separated or defibrated mechanically into pulp for the paper industry.

References

  1. ↑ "PaperOnWeb discussion of brightness and whiteness" . Retrieved 17 September 2007.
  2. 1 2 3 4 Biermann, Christopher J. (1993). Essentials of Pulping and Papermaking . San Diego: Academic Press, Inc. ISBN   0-12-097360-X. OCLC   27173529.
  3. ↑ "Brightness of pulp, paper, and paperboard (directional reflectance at 457 nm), Test Method T 452 om-08". Archived from the original on 15 April 2013. Retrieved 26 February 2016.
  4. ↑ "TAPPI comparison of TAPPI and ISO brightness". Archived from the original on 6 October 2007. Retrieved 15 September 2007.
  5. ↑ "Reviewing the concept of paper brightness" (PDF). Axiphos GmbH. August 2001.
  6. ↑ Ducey, Michael (June 2004). "Matching newsprint qualities to press technology". The International Journal of Newspaper Technology. Archived from the original on 16 August 2007. Retrieved 15 September 2007.
  7. ↑ "Pulp bleaching chemicals information from PQ Corp". Archived from the original on 2 April 2007. Retrieved 17 September 2007.
  8. 1 2 E. Sjöström (1993). Wood Chemistry: Fundamentals and Applications. Academic Press. ISBN   0-12-647480-X. OCLC   58509724.
  9. ↑ "Treecycle Recycled Paper; About Recycling and Recycled Paper". treecycle.com. Archived from the original on 28 January 2020. Retrieved 29 March 2013.
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  13. ↑ Sillanpää, Mervi (2005). Studies on washing in kraft pulp bleaching (PDF) (Thesis). Faculty of Technology University of Oulu, Finland. Archived from the original (PDF) on 9 August 2017. Retrieved 19 September 2007.
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  15. ↑ Singh, R. P. (1979). The Bleaching of Pulp (3rd ed.). Atlanta: TAPPI Press.
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  20. ↑ "Use of Ozone from web page by Air Liquide". Archived from the original on 8 August 2007. Retrieved 19 September 2007.
  21. ↑ Qiu, Z.; Ni, Y.; Yang, S. (2003). "Using DTPA to decrease manganese-induced peroxide decomposition". Journal of Wood Chemistry and Technology. 23 (1): 1–11. doi:10.1081/WCT-120018612. ISSN   0277-3813. S2CID   97876479.
  22. ↑ "Dow Chem. data presented at 1994 and 1996 International Pulp Bleaching Conferences" . Retrieved 19 September 2007.[ dead link ]
  23. ↑ Larisch, B. C.; Duff, S. J. B. (1997). "Effect of H2O2 and DTPA on the characteristics and treatment of TCF (totally chlorine-free) and ECF (elementally chlorine-free) kraft pulping effluents". Water Science and Technology. 35 (2–3). doi: 10.1016/S0273-1223(96)00928-6 . ISSN   0273-1223.
  24. 1 2 3 4 Ragauskas, A. J.; K. M. Poll; A. J. Cesternino (April 1993). "Effect of Xylanase Pretreatment Procedures for Nonchlorine Bleaching" (PDF). IPST Technical Paper Series, Institute of Paper Science Atlanta, Georgia and Technology. 482. Retrieved 20 September 2007.
  25. ↑ Bouchard, J.; Maine, C.; Berry, R. M.; Argyropoulos, D. S. (1996). "Kraft pulp bleaching using dimethyldioxirane: stability of the oxidants". Can. J. Chem. 74 (2): 232–237. doi: 10.1139/v96-026 .
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  27. ↑ Harazono, Koicho; Ryuichiro Kondo; Kokki Sak (March 1996). "Bleaching of Hardwood Kraft Pulp with Manganese Peroxidase from Phanerochaete sordida YK-624 without Addition of MnSO(inf4)". Applied and Environmental Microbiology. 62 (3): 913–917. Bibcode:1996ApEnM..62..913H. doi:10.1128/AEM.62.3.913-917.1996. PMC   1388804 . PMID   16535279.
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  30. ↑ "Dioxins and their effects on human health". World Health Organization. 2010. Retrieved 11 June 2010.
  31. 1 2 "Frequently Asked Questions on Kraft Pulp Mills" (PDF). gunnspulpmill.com.au. Ensis/CSIRO (Australia) joint research (www.csiro.au). 4 March 2005. Archived from the original (PDF) on 2 December 2007.
  32. ↑ "Chlorine Free Products Association". CFPA Today. Spring 1999.
  33. ↑ Duke University; Johnson&Johnson (19 December 1995). "Environmental Comparison of Bleached Kraft Pulp Manufacturing" (PDF). Environmental Defense Fund. Archived from the original (PDF) on 1 December 2006. Retrieved 18 November 2007.
  34. ↑ Ad Hoc Working Group of European Commission (4 May 2006). "Revision of the Ecolabelling Criteria for Tissue Paper: Comments and background to the second draft proposal" (PDF). Archived from the original (PDF) on 6 August 2009.
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  36. ↑ Tarkpea, Maria; et al. (1999). "Toxicity of conventional, elemental chlorine-free, and totally chlorine–free kraft-pulp bleaching effluents assessed by shortterm lethal and subleathal bioassays". Environmental Toxicology and Chemistry. 18 (11): 2487–2496. doi:10.1002/etc.5620181115. S2CID   86517235.

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