Bleaching of wood pulp
Bleaching of wood pulp is the
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.
TAPPI brightness |
ISO brightness |
---|---|
84 | 88 |
92 | 104 |
96 | 108 |
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.
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
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
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
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.
Delignification of chemical pulps is frequently composed of four or more discrete steps, with each step designated by a letter:[10]
Chemical or process used | Letter 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 metals | Q |
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
- 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
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
Oxygen
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
Other bleaching agents
A variety of less common bleaching agents have been used on chemical pulps. They include
Enzymes like
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]
Conventional bleaching using elemental chlorine produces and releases into the environment large amounts of
Dioxins are highly toxic, and health effects on humans include reproductive, developmental, immune and hormonal problems. They are known to be
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
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
- Johan Richter – inventor of the continuous process for bleaching wood pulp
- Paper chemicals
References
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- ^ "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.
- ^ "TAPPI comparison of TAPPI and ISO brightness". Archived from the original on 6 October 2007. Retrieved 15 September 2007.
- ^ "Reviewing the concept of paper brightness" (PDF). Axiphos GmbH. August 2001.
- ^ 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.
- ^ "Pulp bleaching chemicals information from PQ Corp". Archived from the original on 2 April 2007. Retrieved 17 September 2007.
- ^ OCLC 58509724.
- ^ "Treecycle Recycled Paper; About Recycling and Recycled Paper". treecycle.com. Archived from the original on 28 January 2020. Retrieved 29 March 2013.
- ^ "PaperOnWeb description of bleaching sequences". Retrieved 17 September 2007.
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- ^ "Trends in World Bleached Chemical Pulp Production: 1990–2005". AET. Archived from the original on 30 July 2017. Retrieved 26 February 2016.
- ^ 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.
- ^ Starnes, W. H. (1991). Chemistry of Delignification with Oxygen, Ozone and Peroxides. Ann Arbor, MI: UMI Out-of-print Books on Demand.
- ^ Singh, R. P. (1979). The Bleaching of Pulp (3rd ed.). Atlanta: TAPPI Press.
- ^ a b McDonough, Thomas Joseph (January 1983). "Oxygen bleaching processes : an overview" (PDF). IPC Technical Paper Series. 132. Archived from the original (PDF) on 20 February 2009. Retrieved 19 September 2007.
- ^ Johansson, E.; S. Ljunggren (1991). "The reactivity of lignin model compounds and the influence of metal ions during bleaching with oxygen and hydrogen peroxide". Proceedings of the Seventh International Symposium on Wood and Pulping Chemistry, vol. I. Beijing, PR China. pp. 180–187.
- ^ Suss, H. U.; N. F. Nimmerfroh (1993). "Peroxide Bleaching – Technology Review". Workshop on Emerging Pulping and Chlorine-free Bleaching Technologies. Raleigh, N.C.
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- ^ "Use of Ozone from web page by Air Liquide". Archived from the original on 8 August 2007. Retrieved 19 September 2007.
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- ^ "Dow Chem. data presented at 1994 and 1996 International Pulp Bleaching Conferences". Retrieved 19 September 2007.[dead link]
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- ^ a b c d 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.
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- ^ Springer, E. L. (December 1997). "Delignification of Wood and Kraft Pulp with Peroxymonophosphoric Acid". Journal of Pulp and Paper Science. 23 (12): 582–584.
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- ^ Sonnenfeld, David A. (1999). "Social Movements and Ecological Modernization: The Transformation of Pulp and Paper Manufacturing, Paper: WP00-6-Sonnenfeld". Berkeley Workshop on Environmental Politics. Berkeley, CA: Institute of International Studies (University of California, Berkeley). Retrieved 20 September 2007.
- ISBN 0-662-18734-2. Retrieved 21 September 2007.
- ^ "Dioxins and their effects on human health". World Health Organization. 2010. Retrieved 11 June 2010.
- ^ a b "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.
- ^ "Chlorine Free Products Association". CFPA Today. Spring 1999.
- ^ 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.
- ^ 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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