Agriculture Reference
In-Depth Information
Cooking chemistry: action of
sodium hydroxide on lignin
M : middle lamella
P : primary wall
SI : external secondary wall
S2 : central secondary wall
S3 : internal secondary wall
L : Lumen
L
When the temperature reaches 140°C in
strong alkaline conditions, the OH groups
located in
S3
position of the aromatic cores
are partially ionized. A nucleophilic attack
of the
α
S2
carbons generates phenolic frag-
ments. The lignin is then divided into
small soluble units of phenolic type
(Fig. 13.7).
The action of the sodium sulfide as a buffer
helps to maintain a high alkaline medium, but
secondary reactions of degradation are induced
by HS
−
(generated from Na
2
S) reducing the
size of lignin fragments. HS
−
is responsible for
the demethylation of aromatic groups generat-
ing methylmercaptan, MeSH, with a character-
istic smell.
The demethyled groups are then trans-
formed into quinines, leading to the brown col-
oration of the kraft pulp (Fig. 13.8).
Hemp bast fibre cooking yields are around
65-75%, depending on the quality of the deco-
rticated hemp. The resulting pulp is brown and
must be bleached for paper use.
β
S1
P
M
Fig. 13.6.
Fibre macrostructure.
13.4 Pulping Processes
The pulping processes used with hemp are
based on the application of chemicals. Four
main processes can be employed:
•
Kraft process
•
Soda or soda anthraquinone process
•
Neutral sulfite process
•
Acidic sulfite process.
13.4.2 Soda or soda
anthraquinone process
13.4.1 Kraft process
This is the worldwide chemical pulp manu-
facturing process most used for lignocellu-
losic materials. The cooking action is done
at high temperature (150-170°C) using a
mixture of caustic soda (NaOH) and sodium
sulfide (Na
2
S). The sodium sulfide is neces-
sary as a buffer and to prevent cellulose oxi-
dation, because of its reductive properties.
One advantage of the kraft process is the
possibility of recovering all the chemicals by
evaporation and combustion of the residual
liquors, leading to self-sufficiency of the mill in
terms of energy.
The cooking conditions are usually:
1. 15-18% NaOH and 4-6% Na
2
S based on
the dry raw material weight.
2. The chemicals are dissolved in water; the
liquor is 4 or 5 times the raw material weight.
3. A temperature of 170-175°C is maintained
for 1-3 h.
This process is similar to the kraft process,
using strong alkali charges, leading to the
same reactions as described previously.
Anthraquinone is used in place of sodium
sulfide, in this case to prevent the peeling
reaction responsible for the depolymeriza-
tion of the cellulose. During cooking, some
hemicelluloses are also protected and kept in
the pulp.
Anthraquinone is used in small charges as
a catalyst (0.05-0.1%).
This type of cooking leads to slightly
higher yields (1-3%). The unbleached pulp is
quite similar to the kraft pulp.
13.4.3
Neutral sulfite process
The neutral sulfite process uses the principle
of sulfonation of the lignin using sodium sulfite
