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Extractives are complex mixture of terpenes, fats, waxes, resin acids, fatty acids,
phenols and tannins. Most extractives are soluble in alkaline solutions, and a
good solubility permits the processing of wood species that are rich in extractives
(including tropical woods). In kraft pulping, however, high extractive contents of
wood may result in a considerable reduction in pulp yield. This in turn leads to an
4.2 Kraft Pulping Processes 181
increase in the consumption of chemicals, since extractives react rapidly with
alkali and thus the amount of available hydroxyl ions is reduced [126]. The dissolution
of extractives during pulping is of primary importance. Extractives are
responsible for pitch problems in papers, they may also prevent delignification by
covering parts of lignin with resinous material or simply reduce the penetrability
of cooking chemicals into the wood [127], and they add to the toxicity of kraft mill
effluents. The total amount of extractives which can be recovered from pulp mills
varies greatly with the wood species and the storage conditions of the wood
(Scheme 4.22). The highly volatile fraction is called turpentine, sulfate turpentine
or tall oil (from the Swedish “tall” = pine), and is recovered from the digester relief
condensate [128]. The sulfur-containing fractions (mercaptanes) need to be
removed from the distillates.
COOR
COO Na
+
()
n
pulping
()
n
saponification
isomerization
acidification
crude tall oil
extractives
fatty acids resin acids
pitch residue
soap skimmings
destillation
turpentine
-
neutral
compounds
(e.g. sterols)
light oil
extraction
Scheme 4.22 Fractions of extractives obtained after kraft cooking [128].
Fatty acids and resin acid esters are saponified in alkaline pulping and recovered
as tall oil soap [1]. Acidification of the crude tall oil yields the corresponding
free acids. This deacidification process consumes a large amount of sulfuric acid,
which can be reduced by a carbon dioxide pretreatment.
Wood terpenes undergo mainly condensation reactions during pulping, and are
collected as sulfate turpentine. The major reactions of extractive components are
as follows:
_ Fatty acids [129]: these undergo isomerization reactions (the shift
of double bonds in the fatty acid chain from cis to trans, or vice
versa) under alkaline pulping conditions, and are mainly dis-
182 4 Chemical Pulping Processes
solved. Nonconjugated double bonds are transformed to mainly
conjugated isomers. The degree of conjugation is highly influenced
by the prevailing conditions during the cook. For linoleic
acid, almost no isomerization was observed at 150 °C, whereas at
180 °C almost 98% were isomerized [130]. The incorporation of
fatty acids into residual lignin has recently been demonstrated [12].
_ Resin acids [129]: these are also mainly dissolved. Part of the levopimaric
acid (65) is converted to abietic acid (66), though the
extent of this reaction during pulping is variable (Scheme 4.23).
The acidification and heating of sulfate soap finally converts most
of the levopimaric acid [131,132].
COOH COOH
Levopimaric acid Abietic acid
65 66
Scheme 4.23 Conversion of levopimaric acid to abietic acid during the kraft process.
_ Waxes: sterol esters and waxes are saponified much more slowly
as compared to the glycerol esters. Waxes and triglycerides are
hydrolyzed during alkaline pulping; hence, no esters are detected
in sulfate soaps [129]. The sterol esters, waxes and free sterols do
not form soluble soaps as do free acids, and therefore have a tendency
to deposit and as such cause pitch problems.
A number of extractives survive the cook more or less unchanged, and this portion
is referred to as the “non-saponifiable” fraction.
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