Agriculture Reference
In-Depth Information
Combined S•
2
: M
2
SO
3
Free S•
2
: ½ H
2
SO
3
+ SO
2
.
Lignin reactions in sulfite pulping can be
decomposed into three distinct phases: sul-
fonation, hydrolysis and condensation.
Both sulfonation and hydrolysis are
important for lignin dissolution. Sulfonation
makes the lignin more hydrophilic, and
hydrolysis breaks lignin bonds so that new
and smaller dissolvable lignin fragments are
formed.
The hydrolytic carbohydrate reactions are
also important in sulfite cooking. Both cellu-
lose and hemicelluloses participate in these
reactions. But in acid sulfite cooking, the
attack on cellulose is minor, because the acces-
sibility of cellulose is low. Dissolution of hemi-
celluloses through hydrolysis is substantially
lower than in kraft pulping, especially when
cooking is interrupted at high enough residual
lignin content.
The chance of lignin condensation
increases as the sulfite concentration decreases.
Lignin condensation darkens the pulp and can
be avoided by keeping the bound sulfite con-
centration high enough.
produced by the classic pulping mills. The
development of recycled fibres leads to more
and more degradation of the paper properties,
due to the hornification of the fibres, and
increases the need for reinforcement fibres.
These processes can be developed either on
high brightness level in printing and writing
grades using the bast fibre fraction, or in semi-
chemical pulping using the whole plant, as in
the case of the production of reinforcement
pulps for liner board.
13.5.2 Pulping equipment
Due to hemp characteristics such as fibre
length (leading to the production of agglomer-
ates like ropes), the design of the digesters is
different when compared to the one used in
the continuous cooking of wood chips.
Digesters
BATCH COOKING
.
Batch cooking is usually
carried out in rotating spherical digesters
(Fig. 13.9).
The raw material and the cooking liquor
are introduced in the upper part of the digester.
The temperature is regulated by steam injec-
tion. After cooking, the liquor is drained and
the pressure released in order to open the
cover. The digester is then reversed to dis-
charge into a vat.
13.5 Other Processes
13.5.1
Chemimechanical hemp pulping
New developments and recent interest in
paper grades for writing and printing require
different kinds of pulps from those currently
CONTINUOUS COOKING
.
The continuous cooking
of hemp is generally carried out in a screw
1,205
80
80
1,100
Fig. 13.9.
Rotating spherical digester.
