Page 454 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GPB/GAM P2: GQT Final Pages
Encyclopedia of Physical Science and Technology EN013A-619 July 26, 2001 19:32

Pulp and Paper 259

FIGURE 7 Simplified flowsheet of a TMP mill.

V. BLEACHING OF WOOD PULPS the existing chromophoric bond structure to shift light ab-
sorption out of the visible range.
Some wood pulps are used without bleaching for certain
paper grades. However, many end uses of paper require
further purifying or brightening of the fiber furnish. The A. Chemistry of Bleaching
color of wood pulp is usually due to the lignin remaining
Presently, three general types of chemicals are used in the
in the fibers after pulping. In the lignin molecule, con-
bleaching of wood pulp: (1) chlorine containing agents,
jugated single and double bond structures are the pri-
(2) oxidizing agents, and (3) reducing agents. Chlorine is
mary light-absorbing groups (chromophores) responsi-
a major wood pulp bleach. Molecular chlorine can react
ble for the color in pulp. Brightness can be increased
with lignin by either addition, substitution, or oxidation.
in two basic ways: color can be removed by either re-
The lignin is primarily chlorinated or oxidized. The chlo-
moving the lignin or altering the conjugate double bond
rinated and oxidized lignin is much more soluble than
structure.
the original lignin and can be subsequently extracted effi-
In chemical pulping most of the lignin is already
ciently by caustic solutions. In theory, chlorination could
removed. Thus, bleaching is usually accomplished by
provide all the bleaching power. It is advantageous, how-
extracting the remaining lignin. In semichemical or me-
ever, to use only a portion (60–70%) of the total chlo-
chanical pulping, very little of the lignin is ever removed.
rine demand of a pulp in the initial bleaching step. Sub-
These pulps are generally bleached by chemically altering
sequent caustic extraction removes 50–90% of the lignin
depending on the pulp. These steps also appear to alter
the morphological structure and allow milder reactants to
modify the remaining lignin more effectively. Chlorine
also oxidizes carbohydrates so conditions are selected to
optimize the lignin removal. Chlorinations are typically
run at a pH of 2–4, at low temperatures and concentra-
tions (consistencies of 3–4%). Residence times are typi-
callyunderanhour.Chlorineconcentrationscanvaryfrom
3to8%.
Of the oxidative agents, chlorine dioxide (ClO 2 ) is one
of the most effective used to brighten wood pulp. ClO 2
chlorine dioxide is highly specific and bleaches almost
FIGURE 8 The Sprout Waldron Twin Refinery used in TMP Pulp- any type of pulp, other than high-yield mechanical pulps,
ing. [Photo courtesy of Sprout–Waldron Co.] to high brightness levels without significant effect on pulp