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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Bleaching of wood pulp | 2/3 | https://en.wikipedia.org/wiki/Bleaching_of_wood_pulp | reference | science, encyclopedia | 2026-05-05T10:46:11.463710+00:00 | kb-cron |
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. 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.
=== 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.
=== 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. 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. 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, 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. 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.
=== 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.
=== 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. 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. A negative impact of chelating agents, as DTPA, is their toxicity for the activated sludges in the treatment of kraft pulping effluent.
=== Other bleaching agents === A variety of less common bleaching agents have been used on chemical pulps. They include peroxyacetic acid, peroxyformic acid, potassium peroxymonosulfate (oxone), dimethyldioxirane, which is generated in situ from acetone and potassium peroxymonosulfate, and peroxymonophosphoric acid. Enzymes like xylanase have been used in pulp bleaching 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. It is possible that other enzymes such as those used by fungi to degrade lignin may be useful in pulp bleaching.
== Environmental considerations ==