Monday, 28 March 2011 20:13


Rate this item
(1 Vote)

Bleaching is a multi-stage process that refines and brightens raw pulp. The objective is to dissolve (chemical pulps) or modify (mechanical pulps) the brown-coloured lignin that was not removed during pulping, while maintaining the integrity of the pulp fibres. A mill produces customized pulp by varying the order, concentration and reaction time of the bleaching agents.

Each bleaching stage is defined by its bleaching agent, pH (acidity), temperature and duration (table 1). After each bleaching stage, the pulp may be washed with caustic to remove spent bleaching chemicals and dissolved lignin before it progresses to the next stage. After the last stage, the pulp is pumped through a series of screens and cleaners to remove any contaminants such as dirt or plastic. It is then concentrated and conveyed to storage.

Table 1. Bleaching agents and their conditions of use



of agent (%)




Time (h)

Chlorine (Cl2)







Sodium hydroxide (NaOH)







Chlorine dioxide (ClO2)







Sodium hypochlorite (NaOCl)







Oxygen (O2)







Hydrogen peroxide (H2O2)







Ozone (O3)







Acid washing (SO2)







Sodium dithionite (NaS2O4)







* Concentration of fibre in water solution.

Historically, the most common bleaching sequence used to produce market-grade bleached kraft pulp is based on the five-stage CEDED process (see table 1 for definition of symbols). The first two stages of bleaching complete the delignification process and are considered extensions of pulping. Because of environmental concerns about chlorinated organics in pulp mill effluents, many mills substitute chlorine dioxide (ClO2) for a portion of the chlorine (Cl2) used in the first bleaching stage (CDEDED) and use oxygen (O2) pre-treatment during the first caustic extraction (CDEODED). The current trend in Europe and North America is towards complete substitution with ClO2 (e.g., DEDED) or elimination of both Cl2 and ClO2. Where ClO2 is used, sulphur dioxide (SO2) is added during the final washing stage as an “antichlor” to stop the ClO2 reaction and to control the pH. Newly developed chlorine-free bleaching sequences (e.g., OAZQP, OQPZP, where Q = chelation) use enzymes, O2, ozone (O3), hydrogen peroxide (H2O2), peracids and chelating agents such as ethylene diamine tetracetic acid (EDTA). Totally chlorine-free bleaching had been adopted at eight mills worldwide by 1993. Because these newer methods eliminate the acidic bleaching steps, acid washing is a necessary addition to the initial stages of kraft bleaching to allow removal of metals bound to the cellulose.

Sulphite pulps are generally easier to bleach than kraft pulps because of their lower lignin content. Short bleaching sequences (e.g., CEH, DCEHD, P, HP, EPOP) can be used for most paper grades. For dissolving-grade sulphite pulps used in the production of rayon, cellophane and so on, both hemicellulose and lignin are removed, requiring more complex bleaching sequences (e.g., C1C2ECHDA). The final acid wash is both for metal control and antichlor purposes. The effluent load for dissolving-grade sulphite pulps is much greater because so much of the raw wood is consumed (typical yield 50%) and more water is used.

The term brightening is used to describe bleaching of mechanical and other high-yield pulps, because they are whitened by destroying chromophoric groups without dissolving the lignin. Brightening agents include H2O2 and/or sodium hydrosulphite (NaS2O4). Historically, zinc hydrosulphite (ZnS2O4) was commonly used, but has been largely eliminated because of its toxicity in effluent. Chelating agents are added before bleaching to neutralize any metal ions, thereby preventing the formation of coloured salts or the decomposition of H2O2. The effectiveness of mechanical pulp bleaching depends on the species of wood. Hardwoods (e.g., poplar and cottonwood) and softwoods (e.g., spruce and balsam) that are low in lignin and extractives can be bleached to a higher brightness level than the more resinous pine and cedar.



Read 8240 times Last modified on Wednesday, 03 August 2011 23:21
More in this category: « Pulping Recycled Paper Operations »

" DISCLAIMER: The ILO does not take responsibility for content presented on this web portal that is presented in any language other than English, which is the language used for the initial production and peer-review of original content. Certain statistics have not been updated since the production of the 4th edition of the Encyclopaedia (1998)."


Paper and Pulp Industry References

Canadian Pulp and Paper Association. 1995. Reference Tables 1995. Montreal, PQ: CPPA.

Food and Agriculture Organization (FAO) of the United Nations. 1995. Pulp and Paper Capacities, Survey 1994-1999. Rome: FAO.

Henneberger, PK, JR Ferris, and RR Monson. 1989. Mortality among pulp and paper workers in Berlin. Br J Ind Med 46:658-664.

International Agency on the Research of Cancer (IARC). 1980. Monographs on the Evaluation of Carcinogenic Risks to Humans: Wood, Leather and Some Associated Industries. Vol. 25. Lyon: IARC.

—.1987. Monographs on the Evaluation of Carcinogenic Risks to Humans, Overall Evaluations of Carcinogenicity: An Updating of IARC Monographs. Vol. 1-42 (supplement 7). Lyon: IARC.

—.1995. Monographs on the Evaluation of Carcinogenic Risks to Humans: Wood Dust and Formaldehyde. Vol. 62. Lyon: IARC.

International Labour Organization (ILO). 1992. Social and Labour Issues in the Pulp and Paper Industry. Geneva: ILO.

Jäppinen, P. 1987. Exposure to Compounds, Cancer Incidence and Mortality in the Finnish Pulp and Paper Industry. Thesis, Helsingfors, Finland.

Jäppinen, P and S Tola. 1990. Cardiovascular mortality among pulp mill workers. Br J Ind Med 47:259-261.

Jäppinen, P, T Hakulinen, E Pukkala, S Tola, and K Kurppa. 1987. Cancer incidence of workers in the Finnish pulp and paper industry. Scand J Work Environ Health 13:197-202.

Johnson, CC, JF Annegers, RF Frankowski, MR Spitz, and PA Buffler. 1987. Childhood nervous system tumors—An evaluation of the association with paternal occupational exposure to hydrocarbons. Am J Epidemiol 126:605-613.

Kuijten, R, GR Bunin, and CC Nass. 1992. Parental occupation and childhood astrocytoma: Results of a case-control study. Cancer Res 52:782-786.

Kwa, SL and IJ Fine. 1980. The association between parental occupation and childhood malignancy. J Occup Med 22:792-794.

Malker, HSR, JK McLaughlin, BK Malker, NJ Stone, JA Weiner, JLE Ericsson, and WJ Blot. 1985. Occupational risks for pleural mesothelioma in Sweden, 1961-1979. J Natl Cancer Inst 74:61-66.

—. 1986. Biliary tract cancer and occupation in Sweden. Br J Ind Med 43:257-262.

Milham, SJ. 1976. Neoplasias in the wood and pulp industry. Ann NY Acad Sci 271:294-300.

Milham, SJ and P Demers. 1984. Mortality among pulp and paper workers. J Occup Med 26:844-846.

Milham, SJ and J Hesser. 1967. Hodgkin’s disease in woodworkers. Lancet 2:136-137.

Nasca, P, MS Baptiste, PA MacCubbin, BB Metzger, K Carton, P Greenwald, and VW Armbrustmacher. 1988. An epidemiologic case-control study of central nervous system tumors in children and parental occupational exposures. Am J Epidemiol 128:1256-1265.

Persson, B, M Fredriksson, K Olsen, B Boeryd, and O Axelson. 1993. Some occupational exposures as risk factors for malignant melanomas. Cancer 72:1773-1778.

Pickle, L and M Gottlieb. 1980. Pancreatic cancer mortality in Louisiana. Am J Public Health 70:256-259.
Pulp and Paper International (PPI). 1995. Vol. 37. Brussels: Miller Freeman.

Robinson, C, J Waxweiller, and D Fowler. 1986. Mortality among production workers in pulp and paper mills. Scand J Work Environ Health 12:552-560.

Schwartz, B. 1988. A proportionate mortality ratio analysis of pulp and paper mill workers in New Hampshire. Br J Ind Med 45:234-238.

Siemiatycki, J, L Richardson, M Gérin, M Goldberg, R Dewar, M Désy, S Campell, and S Wacholder. 1986. Association between several sites of cancer and nine organic dusts: Results from an hypothesis-generating case control study in Montreal, 1979-1983. Am J Epidemiol 123:235-249.

Skalpe, IO. 1964. Long-term effects of sulfur dioxide exposure in pulp mills. Br J Ind Med 21:69-73.

Solet, D, R Zoloth, C Sullivan, J Jewett, and DM Michaels. 1989. Patterns of mortality in pulp and paper workers. J Occup Med 31:627-630.

Torén, K, S Hagberg, and H Westberg. 1996. Health effects of working in pulp and paper mills: Exposure, obstructive airways diseases, hypersensitivity reactions, and cardiovascular diseases. Am J Ind Med 29:111-122.

Torén, K, B Järvholm, and U Morgan. 1989. Mortality from asthma and chronic obstructive pulmonary diseases among workers in a soft paper mill: A case referent study. Br J Ind Med 46:192-195.

Torén, K, B Persson, and G Wingren. 1996. Health effects of working in pulp and paper mills: Malignant diseases. Am J Ind Med 29:123-130.

Torén, K, G. Sällsten, and B Järvholm. 1991. Mortality from asthma, chronic obstructive pulmonary disease, respiratory system cancer among paper mill workers: A case referent study. Am J Ind Med 19:729-737.

US Department of Commerce. 1983. Pulp and Paper Mills. (PB 83-115766). Washington, DC: US Department of Commerce.

—.1993. Selected Occupational Fatalities Related to Pulp Paper and Paperboard Mills as Found in Reports of OSHA Fatality/Catastrophe Investigations. (PB93-213502). Washington, DC: US Department of Commerce.

Weidenmüller, R. 1984. Papermaking, the Art and Craft of Handmade Paper. San Diego, CA: Thorfinn International Marketing Consultants Inc.

Wingren, G, H Kling, and O Axelson. 1985. Gastric cancer among paper mill workers. J Occup Med 27:715.

Wingren, G, B Persson, K Torén, and O Axelson. 1991. Mortality patterns among pulp and paper mill workers in Sweden: A case-referent study. Am J Ind Med 20:769-774.

Workers’ Compensation Board of British Columbia. 1995. Personal communication.