Pitch problems

Farrell, R. L., Hata, K., Wall, M. B. Solving Pitch Problems in Pulp and Paper Processes. Vol. 57, p. 197... 

Triterpenoids occur in hardwood parenchyma resin, and closely related sterols are also present in softwoods (Fig. 5-6). Sterols typefied by the abundant /3-sitosterol, mostly have a hydroxyl group in the C-3 position. They also appear as the alcohol component in fatty acid esters (waxes). Triterpenoids and sterols are sparingly soluble substances contributing to pitch problems in pulping and paper making. Some trees contain polyterpenes and their derivatives known as polyprenols. Betulaprenols, present in birch wood, belong to this category of substances (Fig. 5-7). 

After felling of a tree the content of extractives decreases, and their composition changes because of various reactions. For acid sulfite pulping, the changes are beneficial, and wood is usually stored for several months to minimize pitch problems and to lower the resin content of the pulp. In the case of kraft pulping, storage of wood has detrimental consequences, since yields of both turpentine and tall oil are reduced. 

Wood resin is considerably changed during the pulping process. The fatty acid esters of sterols and triterpenoid alcohols in hardwoods (waxes) are saponified very slowly. Unsaturated compounds, e.g., fatty acids, resin acids, and other higher terpenoids, are polymerized to high molecular weight compounds, which also give rise to "pitch problems."... 

Ageing as well as bleaching oxidises the components, which increase their water solubility. The material will have a higher tendency to stay in the aqueous phase. Volatile extractives evaporate further reducing the extractive content. This is what smells from freshly cut trees. Ageing of wood and chip before cooking considerably reduce the pitch problems. 

Traditionally, pitch problems were chemically controlled by the adsorption and dispersion of pitch particles by adding fine talc, dispersants, and other chemicals. However, this use of chemicals leads to problems with effluent treatment and environmental pollution. Seasoning of the wood before pulping is another method where the wood is left outdoors for several months. However, these affect the pulp s brightness and yield, which results in an increase in capital cost and land use. Enzymatic control has proven to be a successful alternative, where lipases have shown their ability to reduce the triglyceride content (Farrell et al., 1997). Lipase AYL, produced from C. rugosa and from C. cylindracea, has been successfully used in the pulp and paper industry (Singh and Mukhopadhyay, 2011). 

Wood resin is the water-insoluble material that is soluble in neutral, non-polar organic solvents. Large amounts of it pass through mill systems that utilize softwoods and some of the hardwoods. This resin can cause pitch problems, particularly with sulfite pulping and the manufacture of unbleached sulfite pulps and to a lesser extent with mechanical and chemi-mechanical pulping. Problems are less frequently encountered with kraft pulping and usually they are less severe and of a different type. 

The chemical methods to minimize pitch problems aim to control the colloidal resin by flocculating the particles with alum, by adsorption of pitch droplets onto the surface of finely-divided talc, or by dispersing the colloidal resin in suspension so that it is less likely to deposit. One possible way to minimize the potential for mechanical pulps to cause pitch problems is to reduce the resin in the pulp by washing in the pulp mill. 

The ionizable carboxyl groups in the resin ionize completely in alkaline solution to stabilize the pitch dispersion. However, in the neutral conditions in paper mills, the multivalent metal ions, such as calcium or magnesium, when inadequately washed from the pulp, precipitate the resin and fatty acids as soaps that are difficult to dissolve and remove. Within the normal pH range in the paper mill system, aluminum soaps are charged positively and partly stabilize the dispersion and partly enable the soap particles to be drawn to the negatively charged fiber and thus removed from the system with the paper. Care is needed with the addition of alum, which decreases the pH value and increases corrosion, lowers the strength of the paper, and affects other paper properties. Nevertheless acid conditions of pH 4.5-5.5 can considerably reduce the pitch problem encountered at pH 6-8 and, for that purpose, sulfuric acid alone has been added. 

It has been found that pitch problems and papermill slime can be interrelated. A large proportion of the mass of pitch deposits can be made up of microorganisms. The latter attach themselves to various surfaces in the papermill, where they metabolize the pitch and agglomerate and attract further amounts of resinous materials. 

Methods to control pitch problems in kraft mills pulping tropical hardwoods have been summarized (127). 

J.7.2 Speck Formation and Pitch Problems During Pulping and Bleaching... 

Bleaching pulps with either chlorine dioxide (D) alone or with chlorine (C)-al-kaline extraction (E)-chlorine dioxide (D) successively can affect the speck area. As mentioned in the previous section, chlorination makes the resins not only more hydrophobic but also causes them to aggregate more. In some cases, this results in a total speck area of 1.5 to 2.0 times higher than that before chlorination. In contrast to this, chlorine dioxide usually results in a lower speck area (2). The pitch problems of the kraft pulp of Dipterocarpus sp. can be considerably reduced if the pulp is oxidatively bleached at the first stage. If chlorination is employed, the chlorination product of the terpenoid hydroxydammarenone-II (the main component of the extractives in the pulp) adversely affects the appearance of resin spots and color reversion (157). 

Agatsuma S, Tachibana S, Sumimoto M 1986 Studies on pitch problems caused by pulping and bleaching of tropical wood. Part 17. Mokuzai Gakkaishi 32 34-43... 

Allen L H 1977 Pitch particle concentration an important parameter in pitch problems. CPPA Trans Tech Sect 3(2) 32-40... 

Back E 1956 Pitch control by combined alkali and aluminium sulfate addition. Some principles in controlling pitch problems. Svensk Papperstidn 59 319-325... 

Douek M, Allen L H 1978 Kraft mill pitch problems. Tappi 61(7) 47-51... 

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