Page 455 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN013A-619 July 26, 2001 19:32

260 Pulp and Paper

FIGURE 9 The Sprout Waldron waste heat recovery system used in TMP pulping. [Photo courtesy of Sprout–Waldron
Co.]

properties. ClO 2 can completely replace chlorination in Calcium and sodium hypochlorite are both used in the
multistage bleaching, but because of economics, it is gen- bleaching of wood pulp, primarily as a step in multistage
erally used as part of the initial step or in later bleaching bleaching. The conditions used are a pH of 10–11, tem-
sequences. ClO 2 is used in concentrations of 0.5 to 3% at peratures around 100 C for 2–3 hr. Average chlorine use
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temperatures of 40–50 C and consistencies of 12%. The is about 1.5% based on the pulp.
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reactions are held for 2–4hr. Hydrogenperoxide(H 2 O 2 )hasgreatutilityinbleaching
Chlorination or chlorine dioxide treatment is almost al- pulp. It enhances brightness in full chemical pulps after
ways followed by a caustic extraction. Alkaline extrac- multi-stage chlorine-based bleaching steps are completed.
tion neutralizes the pulp and removes the lignin rendered This is performed at an alkaline pH, at slightly elevated
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more soluble by chlorine treatment. Removal of this mod- temperatures (100 C), and with high pulp consistencies.
ified lignin opens up the cell wall and allows for milder Reaction times are 2–3 hr with peroxide concentrations of
delignification to be used in later bleaching steps. Caus- 1–3%.
ticextractionisgenerallyperformedwithmostchlorinated Oxygen has been one of the most highly investigated
chemical pulps at 50–60 with 0.5–5% NaOH for 1 to 2 hr. bleaching chemicals in the last 2 decades. It can be
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Consistencies are usually quite high (12 to 16%) to reduce the lowest-cost oxidizing agent available. However, it
the amount of water and minimize energy requirements. is a nonspecific bleach and special steps must be taken
Prior to the availability of the commercial quantities to protect the carbohydrates from attack. The pulp is
of chlorine, hypochlorites were the primary chlorine- usually acid washed to remove heavy metal contami-
containing chemicals used in the bleaching of pulp. Some nants such as iron and manganese, then treated with a
of the easier bleaching pulps (e.g., sulfite) could be blea- magnesium salt to limit carbohydrate damage. Oxygen
ched to acceptable levels of brightness in one stage. Hyp- bleaching is best carried out at high consistencies at a
ochlorite is a nonspecific oxidizing agent; therefore, its highly alkaline pH. Oxygen bleaching is usually termed
use alone could not brighten most pulps to very high levels the oxygen–alkali stage since both oxygen and sodium
without seriously degrading the carbohydrate fraction of hydroxide are often used in equivalent amounts. The
the pulp. oxygen–alkali stage could occur as part of an alkaline